(I'll assume OP means overpowered)
Again, that depends only on what the player's up against.
An example to end all examples [on this one :) ] is that playing a god isn't overpowered if you're dealing with three other gods.
Yep, if instead of fighting agaisnt a lone marine - which, mind you, is capable to kill a Cyberdemon even with his bare hands...
* Adral looks at Blade...
...he is up to fight squadrons of marines armed with BFGs, we could argue that being a Cyberdemon is dying on turn 2. So the opposition is what's more important, in my opinion.
I proposed being a Cyberdemon because it would be cool, and could lend us to cybernetical improvements as TFoN noted. Also, if a lone marine is scary for a Cyberdemon, imagine how scary can it be for a former human :P.