I have no money atm but my last round of FOSS donations was just picking based on a function of how much I use a piece of software; how much I like it; and how much money I think it already gets. e.g. I didn't donate to the Linux kernel because that's already well-funded, even though I use it every day and like it well enough.

I think my last donations included River (the Wayland compositor I use). For the life of me I can't remember what else there was but I tend to do rounds of donations when I have money to spare and just pick software based on the above criteria.