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One underrated platformer is Robocop 2. Many people don't like it (as it was demonstrated when it was part of the nesruler's contest), but I think it's great and challenging. The creators really made you feel you were controlling a super heavy man-robot without making it unsufferable to play.
EDIT: is there a clear definition of "platformer"? I feel Shatterhand does have platforming bits, but is mostly a sidescroller action game.
I don't get all the love for Robocop 2. I had the game since I was a kid and it was frustrating as hell trying to get all the drugs only to end up doing a hard as hell shooting gallery and go back to the same level over and over again. Maybe I'm missing something but I'd just rather play the first or better yet, play Robocop vs The Terminator on the genesis with all it's bloody goodness.
My personal favorite platformer is Batman. The game just had really kickass music and the wall jump gimmick was awesome. I also really enjoy Rockateer which is basically a clone of it with the same swapping weapons thing going for it.
EDIT: Hell yeah Mark! Monster Party is the shit! I totally forgot to mention that one.