It's all a matter of the skill level of your opponents. But just based on overall strength and playability, your choices are Erb, ub, oak. Erb - a smart player will very rarely die using him as he can escape with swarm (or mist in some cases). He also can have monks, etc, so he's hard to kill. Being able to stun makes him a great teammate and again, bat swarm means he can run or pursue for a kill over distance. And his stun is an interrupt. I recommend playing him as an assassin. See the strategy section for some builds.
UB can do tremendous damage with ooze/spit. Anyone near the beast is losing health due to ooze. When played well, he's a dg killing machine. I'd still lean towards trying to become great with UB, but whatever you find fun. UB has an interrupt as well that holds players for 2 seconds or so. UB can have extremely high hp and is hard to kill especially if you use sigils.
Oak can be extremely strong for autoattack damage, useful for pushing troops, and he has the unique ability with high level shields to not be stunned if you cast a shield on yourself. This means you can lock portals over and over and not be interrupted (once you've captured one initially). This is a pretty big deal if the game goes to giants as you can keep relocking a portal yourself and the other team can't do too much about it. He also has an interrupt, but his can be cast a range.