Man so many people mentioning Blasters...I know I left a long time before CoH was shut down, but did they get gutted that hard? That's rough. They used to be top tier at everything.
My most underrated has to be Archery (and this may have changed by the end). Long range, high dps, low cooldown attacks. Rain of Arrows was monstrous, doing the exact same damage as any other Nova ability, but without taking all of your mana and a lower cooldown. I love Archery so much.
Trick Arrow had a ton of potential I don't think was ever fully realized either. I really wish they had got Oil Slick to work with fire as they originally intended. Heck, they may have and I just didn't know it.