Kingdom of Loathing; nothing else comes close

I've played alot of paperclips in classes, it just looks like a random website but it's a really fun idle clicker thingy it's at https://www.decisionproblem.com/paperclips/index2.html

Learn 2 Fly and Feudalism 2

I replay candybox/candybox 2 at least once a year. Fantastic incremental ascii games

Urban Dead was fun back when I played it. Used to be on the forums a lot and organize curing centres or safe zones.

Dominate Game. Great online risk game with elo system. You could play the default maps in the browser but thier client let you play tons of customer player made maps. Sadly this one got taken down a long time ago, is has a spiritual successor as Dominate 12.

Supremacy 1914. Only played 2 matches of this game, but do to its nature it was over the course of 6 months or so. I believe they have a download client now and the game is also on Steam now.

I had a list of some flash games I used to obsess over, but I can't find it right now. I'll throw Sonny 2 as my recommendation, I beat that game many times and I still come back to it every now and then. It's a top-tier RPG.

I would also recommend Gemcraft and Bloons TD, but they have premium sequels now that are way better than the web games.

Krunker.io

For me this is a hard toss up between Pizza King too and the Submachine series, the Submachine series is an amazing escape/puzzle style series that has a really good story

and Pizza King 2 is the best management style Pizzeria Simulator game that I have found to date, many of those management style games focus more on the cooking aspect than the management aspect but this one takes the cake with being an actual management Sim instead of a cooking

Slither.io is definitely my most played browsergame. It gets way more fun with a mod that lets you zoom out a little, as the bigger you get the worse the experience normally gets..

cookie clicker, that chromium dino game. I think I also did dark room at some point.
that pokemon browser game. There's also a taiko game out there. (haven't checked lately)

I remember spending a lot of time playing browser games from Nitrome.

Mutiny, that cave runner game, Dirk Valentine, Ice Breaker, Bad Ice, etcetera.

I also played a lot of the Bloons series. Hell, when I found out about BTD5 flash, I thought it was some fake fan made game that would give me a virus.

When I was little, it was Club Penguin and Webkinz. Now? Flight Rising

https://kittensgame.com/web/

Man I miss the Flash era.

I spent a ton of time in Kingdom of Loathing. I kept an old laptop that still had a flash enabled browser for a few months to play Bloons Tower Defense 4.

There was this game, I forget the name of it, but you had to build and drive little vehicles to overcome challenges. It was technically amazing for a Flash game, and I'd love to have it back.

Tagpro

I've spent a truly mind blowing amount of time on MissionChief. They don't give exact stats, but I'd say I've played at least 2k hours over the last 4 years

diep.io and only domination. Stopped going when they put that game mode on some stupid rotation.

When I was in school it was all about Neopets and Adventure Quest.

Penguin attack

Age of War. Was also the last browser game I ever played. Yes it's old.

Ita not a single game, but I waste a lot of time on gamesnacks.com

I played a lot of BrowserQuest when it was still hosted by Mozilla. It was a really fun little rpg.

I think this old school browser game Utopia counts?

Played it a ton years ago.

Evolve

Adventure Capitalist, City inc, Reactor Idle.

Adventure Quest, Habbo Hotel, Coke Studios and Tribal Wars

Does tribal wars count?

There was also one about forming your own country and passing laws and stuff but I don't remember the name. It was out around 2002 or so

Played linerider waaaaay too much back in the day.

Amazed no one has mentioned Antimatter Dimensions. It's now on Steam, but was originally on a few sites (I played it on Kongregate). Fantastic, and now with the latest update and the 'aero' skin and hidden games, it's multiple games (Including the original Paperclips) inside one :D