The ability to assimilate everyone else’s powers. Duuuuh.
Of course, that’s only useful if you know for sure that there are other super-powered people in the world. It’s not of much use if you are the only one.
Likewise, there could be issues with assimilating powers from people who can’t control their powers (Cyclops ?) or whose powers involve gross (meaning large, not necessarily disgusting) physical changes (Angel, [pre-transformation] Beast?).
(Now guess who my favorite X-man is. 🙂 )
The Mimic, from X-Men 27-29. (Yes, I read quite a few comics as a kid.) 🙂
The Wolf