At first, anything that was free. Then I found the *arrs, moved to Usenet and never looked back.

I don't have any experience with Usenet or any current DDL sources. I generally check public trackers first, then private trackers, and mostly use private trackers when I can't find something on public trackers.
My selfish reasoning is that with public trackers, I don't have to worry about my seed ratio, etc. I generally seed to a 3.0 ratio, and I do have some older/more niche stuff set to infinity, but when I need to clear some space in my seed box, it's nice to be able to delete stuff without worrying I'll be penalized.
From a less selfish perspective, if it exists in decent quality on public trackers, I'd rather contribute to keeping it alive there, where more people can access it.

If I download something:
I don't care about quality:
Public > Private > Usenet > DDL

If I care about quality:
Private/Usenet> Public (I will download public if the group is the same) > DDL

I can't find it:
Anything that has some part of it. Usually DDL or some streaming site.

Using private trackers for many years and never had a reason to try usenet, but only hear good storries about it. Public trackers are actually backup in my arr apps, but they are almost never used.

Depends mostly on what content you want to load. Public trackers are fine for popular stuff like Netflix shows or Hollywood movies in English, but you run into trouble as soon as you want more nieche things like audiobooks or movies in your native language. For that you need specialized private trackers or usenet indexers, idk which is better though. One requires invites and seeding, the other usually cost a few bugs extra, so it kinda depends what you prefer.

DDL without a debrid service is pretty bad imo, you'll constantly find things on hosters you don't have a premium accounts with. Even with a debrid service it's hit or miss, especially since links on some hosters get DMCAed very quickly. It's ok for one-off downloads every now and then, but the fact that there isn't even an option to automate the process is pretty telling.

I switched to xdcc

Usenet. You can download files at high speeds and don't have to fulfill any requirements. But it's insanely hard to get into the top usenet indexers.

Usenet > DDL > Private > Public

Usenet is the easiest to automate. I can set it and forget it. For DDL, I use two private sites so I know the quality of what I'm getting. Private I need to worry about ratio, but quality will be more controlled than public.

public tracker I can easily search directly from qbit, also good for newest releases

Pahe for DDL stuff, if I want better X265 quality at small size (PSA used to be good, but they fucked up something and I can't play their X265 rips anymore on TV, X265 Pahe rips always work)

ain't gonna pay for usenet or getting invited to private tracker

DDL

Private trackers. Better quality assurance than public trackers, free unlike usenet.

I prefer DDL as first priority as I get majority of my content from there. I used to torrent from public trackers when I started torrenting many years ago. A year ago I was able to get into some private trackers. I use them to upgrade my movie collection i.e replacing my old files with better quality.

Public trackers for filesize over quality and modern codecs like av1 or x265. Private trackers for harder to find stuff or good quality over filesize.

Private trackers.

I have private trackers and Usenet and my quality profiles in the *arrs mostly always pick from torrents. I will cancel my Usenet subscription. I also kinda like the grind of private trackers just for the heck of it.