If we're talking about things I use regularly, probably my desktop computer. I bought it at an auction. This was in '04 and have been upgrading it one piece at a time ever since. I'm currently gathering parts for another "gut it and start over" level rebuild and I think the only original parts at this point are the case and the floppy drive.

If it's just things I own and don't use, I do technically own a '76 Chevy pickup that is almost infinitely repairable but that hasn't had an engine since the early 2000s

My tractor. It's the ship of Theseus at this point.

My phone. I'm certified in repairs for it.

I've got the robot vacuum of Theseus here... early, 2nd or 3rd gen Roomba I guess it is, I've probably had since 2009 or so I think. I've replaced damn near all of it, and I'm ALWAYS shocked when something new breaks, I check online, and I can get a part and replace it! And often times the replacement ends up being an upgrade too. I think it must have come from the Twilight Zone or something, I don't know.

Guitars. I have more than enough, but I still cant resist a good deal on a dirty, old, neglected vintage guitar. Clean it up, fix up any problems, re-string it, play it for a while, and sell it. But if it's better than something I've already got, then I'll keep it and sell the old one.

I'm slowly building and improving my stable, without spending much money at all. It means my guitars probably won't say Martin, or Taylor, or Gibson, or Fender, but they'll all be excellent guitars anyway.

I'm also good with dryers. There isn't anything on a dryer I can't fix. They can almost be a Ship of Theseus situation.

My Thinkpad:

Perhaps inflating it with upgrades, but very few original parts remain on my ThinkPad X230. Swapped parts include the motherboard, RAM, SSDs, wireless card, fingerprint reader, keyboard, LCD panel, speakers, cooler, battery, screws, bezel, palmrest, and hinges.

The multi-tool pen in my pocket everyday carry too. Originally had ruler markings, Phillips and slotted bits, stylus tip, and a level. Didn't need the level and wanted it shorter so it fit in my key pouch, so I took all the parts out, sawed off part of the barrel, and put it all back together, sans level. Refills are easily available online or can be crafted simply by popping the ink stick out of a regular ballpoint pen and cutting it short.

My 90s Chevy is also very repairable and the parts are still very plentiful. In no mood to get something newer, especially after seeing the engines and touchscreens of some my friends' newer cars.

I'm pretty good at repairing computers, but please don't tell my friends and family.

Probably the 72 year old Ferguson tractor I own, new parts are so easy to find and usually very reasonably priced. It's also designed to be easily user serviceable and user repairable.

My crown Victoria. I've had it for 15 years and do all the repairs on it myself. Sometimes it's difficult to get to a part cause of a stupid design, but I usually win in the end.

For a unique answer, I'm going to say my model trains from the 1940s & 50s (Lionel, O gauge). They were designed to be taken apart & serviced, and it shows. And they have enough common parts that even though they're now 75+ years old, you can still get parts.

I'll also throw my 1997 Ford F-150 into the mix as a more common answer. Ford made literal millions of these, so not only are parts still available, they will be for decades still to come. Heck, I replaced the motor last year, and was able to get most of the bolt on parts I wanted straight from the dealership. Not bad for 25+ years old.

I have a few.

Blunt umbrella - all the parts are replaceable, big win.

3d printer - I am confident I could replace any part that dies. It's not the newest or the best, but it's mine.

Also our humidifier is, in theory super fixable, a venta air, but it's been rock solid and hasn't needed work yet.

3d printer. Just wish I didn't have to do it so often.

Most repairable thing I have is probably my truck. It was made in 2007, before they started to take away user serviceability.

Oh also I have a bunch of old computers that are very repairable. I mean, I would need the right components, and I can’t make those myself, but if I could source the components, they’re really easy to repair. Probably the hardest thing to repair would be the sheet metal.

Probably hand planes, I can easily keep those going for decades myself.

Some of my furniture I guess, I’ve already rebuilt the bookshelf that fell apart.

I don't have it anymore (sadly), but for a while I owned a Hitachi TX200 projector.

All of the fans were standard PC fans and easily replaceable. The polarization filters came on little extra modules, you could take them out, replace them and realign them easily. The service menu had an option to shift the pixels for each individual LCD to counteract for pixel-drift and you were also able to calibrate the colors for multiple zones. It's a great projector and can run for many years with a few replacement parts.

Sadly, it was only 720p and Hitachi never made a higher-resolution model. :(

Bicycle

My Rancilio Silvia espresso machine. Easy to take apart and replace anything. Parts are readily available from many sources. Also, my BMW 3 series, but I’m a BMW tech, so I better be able to fix anything on it. But it’s 14 years old and is like working on a horse cart compared to the current ones. Luckily BMW are designed to be able to fix anything on them, unlike most other brands. Don’t even get me started on Tesla, what absolute junk.

Mine is probably a tie between my Arrma RC cars and my KLR 650. It's a 2022, but even then it's easy to service since the design didn't change much and parts are easy to come by.

my cheap ass salvaged speakers. I got them from a friend who got them from her father who got them from a school janitor who got them from a school that was throwing them out. Each of those step did some "repair", until they got to me and my roommate, and we undid every previous "repair". we are still using them

Possibly my dingy. It's a eighties fibreglass boat. I'd feel super comfortable swapping any part of the rig and doing light repairs on sail or hul. Although, if the mast broke or the hull was badly damaged I would probably have to let it go ):

Also, on the topic of boats. That one isn't my boat, but I somewhat frequently sail a stjärnbåt, a type of smaller wooden boat. There we have a real ship of Theseus situation, I think there's no part of that boat we couldn't repair.

Nearly everything is repairable as long as you invest your time for it.

Since my background is in PC repair... my PC.

cast iron and carbon steel pans

I have a reusable bag. I repaired the seams at least 6 times.
I really love the bag, I got it from CES about 7 years ago. And even it's an evil entity now, it is hands down the best reusable shopping bag I own.

My mid 2000's Japanese SUV. Manual transmission, regular old v6 gas engine, only one "computer". Same engine, drivetrain, and frame for 3-4 different models so parts are abundant. Can repair/replace anything on it with a handful of power tools and elbow grease.

It might be my pickup truck.

It might be my toilet. Every moving part in the thing can be bought at every hardware store in town.

My washing machine is pretty good, it's a Kenmore 80 series, it'll probably outlast me.

Me.
A lot has and continues to go wrong, but mostly self-repairing. At some point some failure will be unrecoverable, but hopefully that's a long time in the future.

An even better answer might be my cats. The small number of repairs that need professional assistance are cheaper than the same issues would be for myself. However I don't expect them to last as long, unfortunately. Even though I got them very nearly brand new a few years ago.

Do they stink? Like, old vacuums got stink like the shit they've been sucking up for years. How do you fix that?

Not my fucking 40 year old knees, that’s for sure.

I agree with @Tolookah@discuss.tchncs.de the 3D printer is also very repairable.

My tools. I've amassed quite the arsenal of hand and power tools from 1840-1970. I refurbish and rebuild them into much higher quality workhorses than you can get these days for a fraction of the cost. Even if the price of modern tools wasn't of any concern, outside of two very premium niche manufacturers, you literally can not get good tools anymore. Nobody makes them. Home improvement stores are full of poorly designed, low quality garbage for people who have never used an actually good tool before. No one has made a made a good combination square in so long that most people have never used one. Chisels and saws are a goddamn tragedy. Power tools are all run with chips than burn out, are covered in plastic guards that break or melt, and are running entirely on brand favoritism from people that don't know they've been had. My table saw is from 1953. It cost me 40$ and an hour of sanding rust and tuning. It has one mechanism and will eat through anything. My band saw is from 1968 and cost me 60$, plus 28 for new guides and tires. My favorite chisel is from 1884, and cost 5$. I still can't find one I like nearly as well in any other size. My favorite block plane was 6$ and an hour of tuning. It's from 1878 and kicks the hell out of the 40$ Irwin dogshit I picked up before I knew better. My panel saws have been used hard for 160 years, and will not only outlive the disposable garbage from home depot, but will do a better job and outlive me.

I've made a hobby of bringing anything I can find at thrift stores back to life. It prevents waste, and keeps a tool that had real care put into it's development from ending up nailed to the wall in applebees. As a bonus, collectors generally hate refurbished tools, and I hate someone removing things from the shrinking pool of good, cheap tools so they can put it on a shelf or try to sell it for hundreds as a rarity.

Strictly speaking here, something that could be fully repaired by the actual tools I have without just swapping for new purchased replacement parts would probably be one of my guitars. I could essentially make every piece of them from scratch if needed except maybe the pots, caps, and truss rod. I could probably repair the truss rod but making one from scratch would be tricky. If I had an acoustic guitar, that would be an easy answer.

It might not turn out well, but I could make it playable again.

I could repair anything on my bike, and kayak. I might be able to rebuild my snes on a component level.

If you allow replacement parts, almost anything can be repaired of you are willing to spend enough on it.

My PC in terms of being able to swap everything out, at least while parts are still available, I won't be surprised if the RAM shortage was an attempt to try to kill parts sales for PCs at some point.

I mean, half of my hammers have had their handles remade with whatever hardwood was closest. So I'm gonna go with those.

However, the most complex thing that I can repair with sourced parts would be either my truck, or my truck. I've had to fix it with "close enough" parts before, and I will fix it until the frame rusts (which given Northern states and salted roads probably won't be more than a few more years)

There's a smattering of electronics that I've seen mentioned multiple times as well.

One thing that is theoretically repairable, but is hard to find parts for, my washing machine. It's a old 50s model with a lever to engage the motor, a roller on top to squeeze water out, and a simple steel gear system to drive everything. I should be able to open it up with a crescent wrench and flat head screwdriver, but I doubt I will ever be able to find ready-made parts (I have looked briefly when I thought I had a problem with it that sorted itself) but I might be able to bodge some things together if needed.

My 3d printer is half 3d printed and a fourth recycled garbage. I've laid hands on every single part in her and hopefully soon I'll replace the box as a whole with a vintage tv cabinet and finally put a 30cm heated bed in her. The only thing I can't replace is the thermistor because I don't know how to program for a modem one.