I budget my donations so I make an effort to see who I think need it the most. For example, I use Tor daily, but they have huge institutional funding. My to-do list app doesn't.

There are also some worthy candidates who simply reject donations, like Handbrake.

A few I haven't seen mentioned:

• Small websites anyone can use for FOSS services, like (e.g. Private.coffee, Disroot, Nadeko, Riseup)
• Any of the 5 remaining Invidious instances, Google has put effort into killing off other instances. Same with other social media alternative frontends.
• Someone mentioned F-Droid, I don't know what I'd do without the Google Store alternatives like Aurora.
• yt-dlp devs
• Lemmy and various instances
• Your operating systems, incl phones and servers if relevant
• Codeberg
• A few FOSS softwares used for non-fediverse sites I use. Look at all the sites you use and think about which ones are probably underfunded. Don't be afraid to ask if they haven't said anything.