My Bots/Traps MM was the quickest to 50 for me, though not the most fun. That would be my Storm/Ill Defender, she has such a bag of tricks there is almost always something she can do. Good on teams, too, at least once the PUG realizes they didn't hire a HEALer, they hired a DEFENDer.*
* Favorite feedback at the end of a TF during which nobody but the bad guys took much damage.
"Good heals, BTW!" Didn't have the heart to tell them...