My picks:
Most overachieving: Batman
Reason: He's a mere mortal. A product of extreme tragedy. Could've led a life of luxury and yet he chose a path of pain and loneliness for protecting the innocent. He hangs with gods (JLA) and is respected by them as being one of its most important members.
Most underachieving: Terrax
Reason: With the power cosmic, he really has done very little terrorizing.
^cap is an avatar of the "real source" aka american patriotism *hums star spangled banner*
anyway,
overachiever (other than the obvious choice of batman) = lex luthor
underachiever = the snowman made of dog-piss snow named DARKSEID. the dude just needs to take up boxing and he would be unbeatable, or at least block.
overachiever= wolverine. for a 5"3 shorty, hairy guy with probably a bad case of BO and only crankiness to run on, he sure gets to beat a lotta guys he shouldn't be
underachiever= Jesse Custer from Preacher, the guy has the coolest power in the world but spent the whole time eating his girl friend's holy-of-the-holiest
~Sado