Heyoo, a lot has been said by these wonderful people ^^ but imma just give my 2 cents of advice anyways : )
It all just depends on what you like and enjoy!
I main parkour because I like the arcade, quick playing nature of it, as well as kitpvp, though kit is more aggressive and quicker.
If you're into aggressive games, clearly kitpvp and factions!
If you're into mining and grinding farms and afking, Olympus (Atlas / Aether) and then just choose one with the community you best like (or both!)
If you're into just traditional minecraft (but with twists and perks), then survival is good!
If you're into skyblock type gamemodes, check out Aztec or Oasis (identical, just different communities), or Islands!
If you just want to have a nice quiet time and like to build, then definitely creative, of course!
PS. I have only played a tiny bit of each gamemode (apart from parkour) so please correct me if I'm wrong on any of these ^-^
But just send a little bit on all of the gamemodes, you'll soon find out what you like and dislike! You don't have to main just one, you can play all if you really want to! The server is your oyster (or whatever the saying is) :D