i'm still playing GDI campaign (last mission), but i'd definitely say mammoth tank!! awesome unit with the rail gun upgrade, especially when it's promoted to the star rank... almost unbeatable...
2nd would be the zone troopers... a mini army of 10-15 zone troopers can raise a lot of hell...
and last of all, the commando dude
stick him inside an APC, and he pretty much takes care of all those ground infantry rushes at you...