Maggie all the way. Three reasons: Violence. Speed. Momentum.

I love her ability to force people out of cover. I love that she is the best with a particular weapon class, to the point of changing/increasing the viability of weapons like the EVA-8. It's great that she's a hard counter to Gibby and a soft counter to control legends. Oh, and she's one of the few post-launch characters with dialogue that isn't annoying as shit.