Mine has to be skyblock. Everyone is really nice even though there are occasional spammers or trollers. I also just really enjoy playing skyblock with my friends and everyone on this server. On another server, my answer might be different but the atmosphere on this server makes it so much fun.
Towny survival is also my favorite, although I play all. Towny is so welcoming and you are never really slid under the table there. I've seen on other servers how people that are non-donator are sometimes completely ignored.. >.< but on survival no one is ignored! No one is really un-important! It is rather great!
I'm a big fan of SkyGrid I think it's so unique and not many servers have it, but i do also love Factions even if it stresses me out sometimes
My favorite? Skyblock~ And factions. I love the ability to build my own island in skyblock, and factions? Going out where theres nothing and building my own little hunkered down base is fun, nobody finds me.