I recommend the defender for beginners, or even the bio-engineer. Honestly, the champion can be spread too thin for my tastes, and the commando and berserker are so focused, you will probably die if not prepared to deal with their weaknesses.
For me, the game really became fun when done co-op. My buddy and I went with a Commando and Berserker and had a blast getting them up to level 50. Now, we're grinding through the game for the level 50 armor sets...
