Lately, I've been on a spree of trying as many gamemodes as I possibly can. If it works at the moment, I'll install it and give it a go. Here's what me and my friends over at LP tried;
Zombie Survival: lots of fun, gets repetitive quickly though.
DarkRP: Was fun to add little snippets of code to add new features. Surprisingly easy to populate, maybe that's just because I can get a server full just by posting on our community forums.
GarryWare: Fun! For a short while.
Prop Hunt: Ditto!
I think that's all I've tried as of yet. Suggestions for more fun times? :)