Either Skyblock or OP Prisions.
Both of those are my favorite gamemodes on Manacube, and the ones I played on old ones before I found manacube.
For some who don't know, I code some of my own Minecraft server plugins for fun, that I like to tinker around with in-game. I have my own little test server for myself that I run on localhost. If you don't know what that is, its basically just a server set up for me only, not port-forwarded, or anything like that. This test dummy of a server that I run every few weeks has basically every gamemode that I could think of, which makes it quite crazy. Because of that, I've had experience with running/playing on all of the different gamemodes that have come to mind for me in the last two years.
Back to my point, I'd say Skyblock or Prisions because of the fact that they have so much creativity, without having the threat of dying or having to restart because of things out of your control. You don't have to worry about claims (survival), other people raiding you (Factions), or raging out of your mind (Parkour). Creative would also be a fun gamemode to have, so in the end, one of those three would be my picks.
But alas, we have Manacube! Thank the gods!
-Wings