>>712798914horizon is the shit one that's shilled hard. its OK but really overrated, main thing it has is a big pop but too many ppl in one server kind of ruins XI cuz you run into a shortage of exp camps and too much competition at bosses.
nasomi is trash.
eden is OK.
era was my favorite but its a smaller community, better if you're busy in RL cuz FFXI is the kind of game that you normally need minimum 6 hours of uninterrupted free time to have any time to play.
there are/were some other servers but a few have died and I'm not up to date with any new ones.
My advice is pick whatever one appeals to you, but go with one that's been around at least 5-10+ years. FFXI private servers arent quite as bad as Ragnarok ones, but new ones are pretty known for dying a year or two in out of the blue over some retarded drama.