Phew, a lot of mechs fit this question for me. I've always liked seeing what I can get out of sub-optimal mechs, and have had surprisingly good results out of them many a time .
Charger: Yeah, it has popgun armament, but it is good in it's intended heavy recon role. It makes a great physical attacker though, and being cheap as chips can handle the hand to hand while more suitable mechs lay down the firepower.
Quickdraw: Again, not the best mech on paper but it does work well for heavy recon and physical attacks.
Crusader: Always favored the 3k, but even the 3r has done well for me in the past, as long as it sticks to missile support and dumps its ammo when appropriate.
Dragon: The Ac5 is a turn off, but if you take advantage of its cool-running nature you have a good skirmisher that can consistantly provide long range support on the move.
Whitworth: Only really 'bad' in the fluff, actually a great medium mech.
Cyclops: Compared to the Atlas, it's pants, but pretend it's a heavy and presto, you have a pocket Atlas. I've had it perform well as a bodyguard for my comander's Warhammer many a time.
Honorable mention: The Loki. Yeah it's not great. However when you've never seen Clan mechs before and one pops over a hill and slags your prize Warhammer at age 12...well yeah that was surprising.