TU beta is the best game type because it kinda brings you back to like halo 3 and fixes the annoying things about reach
Invasion is good and fun to play ,gives good credits and you can be UNSC and covenment in the same match.
team swat is good if your not to bad at shooting people in the head ,and if you cant get them in the head its good practice.
team arena is helpful for your halo charts skill and medals but not easy to play.
multi-team is good if you have a bad k/d spread (kill/death) and nice credits.
action sack is a fun game type if you don't want to play seriously online.
MLG is for serious gamers and is good to help your skills improve.