It's actually a tie between Survival and Skyblock, they've both got a wonderful community (mostly lol) that help each other and I've met like 3 people on Skyblock in one session of playing (like an hour) that I got on really well with!
As for Survival, I'm just so used to playing on it, you start to like something once you've got the hang of how everything works :3 How about you though? What gamemode do YOU like? <3