I'm on SDF for its no-defederation policy and because SDF outside of Lemmy has a proven track record of public service and longevity. Like SDF is much older than I am. Occasionally there are hiccups and the SDF team is a bit slow to respond, but otherwise it's been a good time. And the people here seem pretty chill too.
For my needs it's the best one. But you might want something with a curated feed, a particular political orientation, a smaller or less crowded instance, or something else.
the advice i've heard is pick one that isn't too big but not too small either. spend some time looking at the local communities to see if they align with what you want. .zip is focused on gaming and tech which is me. others are politics heavy. some are widely defederated, which may be the only thing that really matters-- you don't want one that everyone can't stand (.ml, etc)
I don't love their decision to defederate hexbear, but otherwise I like this instance.
Not to say I don't understand their reasoning for defederating, because I do, I just prefer as little defeds as possible aside from corporate influence (fuck threads)
If you are into solar punk slrpnk.net has been amazing for me. Due to some weird timing with the admin being out of town when the servers went down they're down right now for a bit, so I'm not sure when they'll be back up and running. But the current situation is a hiccup, not a consistent issue. I love the server, and if anything, the community reaction to the admin being unable to fix this for a few weeks has made me love it even more.