Out of the three, I prefer Flatpaks. Mainly because they have a nice centralized-capable model for performing updates (but not locked centralized like Snaps are), and I can't say I've personally run into a distro where Flatpaks didn't work.

I haven't taken a look at them from a developer standpoint, but from what I hear the development experience isn't bad? If that isn't the case though, I'd love to hear more about why.