The idea of underpowered or overpowered ATs is laughable. This isn't those other MMOs where dev nerfs or buffs rotate on quarterly basis (ESO, looking at you).
ATs are a playstyle. A playstyle is neither overpowered or underpowered. It just is your approach to solving conflict. Now, that said, CoH mostly appreciates a blunt-force hammer to the face approach to solving problems, hence ATs with high DPS with strong AoE (blasters) can feel like they are performing over the midpoint. Sure, if you measure your personal satisfaction from pure dps, then Blaster (or Scrapper or Brute) is your go-to OP AT.
But it is the actual indicator for OP or UP? Nope, that's your powersets. Not all Brutes are equal. Fire/BIo and Kinetic/Super Reflexes are two totally different beasts. And those same power sets in a different AT are going to perform differently as well.
Masterminds as C tier? Sure, if you like AoE spamming mobs. But MMs have proven able to tackle GMs, AVs, farms and everything in between. So, actually, have all ATs. Just when you think no one can take a set of powersets and solo an Imperious Task Force, someone posts a vid doing it on a Psi/Emp Defender.
So don't ask which ATs are performing -they all perform well enough - even Sentinels (which are gernally rated as 'mediocre'). But its the power sets that are gong to make or break your decision about how well powered they are - well, that and your gameplay. I cannot take a Emp Defender and solo an ITF, but there are people who can do it just fine.
Trick Arrow, for example, used to be a miserable set. I found it only worked functionally back on Live when i paired it with Illusion Control, where it did reasonably well. Mid-tier. But with another AT? In the basement.
But now, after the fixes to Trick Arrow, I'd call it A tier on my Defender. A+ on my controller, and B+ on my Mastermind. Its not the AT, its the powers. And the player.
For anyone reading this who still cares enough to have a list of easy choices for leveling and getting into the game, here's my list of 'OP':
Demons/Time mastermind.
Time/Dual Pistols mastermind.
Fire/Super Reflexes sentinel
Ice/Ice blaster
Willpower/Martial Arts tanker
for starters.
Enjoy.