Actually, Cabin Fever is the fastest, as you get 300-500GP and 200-300EXP PER game.
Dude Cabin Fever will take you a while to finish.
CTF Death Room is faster.
Join a pro team and you will finish a game in 1-2 minutes and get like 100+ EXP.
That's a lot.
Cabin Fever around 40 mins.
100*40= 4000 EXP