What is the rarest move in chess. I think it might be promoting to a rook. I just recently did exactly that so I wouldnt stalemate my opponent. But how often would that ever happen ?

Any move you can think of that is rarer ?

axc4 is very rare. So is Nv15.

I like that. 0-0-0# !!

Na1xc2# this requires three Knights of the same colour to be on the board, one on a1, another on a3 and a third on c1.  I threw in the capture and checkmate for good measure.

Well this one is forced from the Queen sac and is slightly varied from a game between Lasker and Thomas, enjoy!

Love that game -- my only dissapointment is that O-O-O# wasn't played.

I once had a game which ended with me (black) castling and white resigned.

As for the most rare move, excluding some situations like TheGrobe's example in post 6, and limiting ourselves to "categories" of moves, how about these:

1. Discovered checkmate where the rook moves
   
2. Underpromotion checkmate
3. en passant checkmate
4. underpromotion followed by resignation
5. (not so rare, but fun) Checkmate in the middle of the board. (e4/d4/e5/d5)

En passant resulting in discovered checkmate.

A smothered mate in the middle of the board also has to be exceedingly rare.

A seven-way fork is also possible, but likely quite rare.

I would think this is more common than en passant resulting in non-discovered checkmate.

Possibly -- particularly when the pawn push that enables en passant is the only defense to checkmate on the previous move.

Zugzwang helpmate

!

Discovered double check (I can't think of any way for this to occur except via en passant).

How about forced underpromotion to avoid a stalemate?

I think the rarest move ever was the castling at Tim Krabbé's problem in 1972.

See this thread: http://www.chess.com/forum/view/more-puzzles/joke-chess-problem?lc=1

This was a loophole in the castling rules at that date.

hmm, of all the underpromotions I think underpromotion to a bishop is easily the most contrived and it's probably never happened as the best move in a serious game. 

I believe that there are scenarios where under-promotion to Bishop is necessary to avoid a stalemate, but it certainly qualifies as being extremely rare.

yes, there are positions where underpromoting to bishop is the best move, but they are very contrived indeed.  I've not seen a single game where it has been required. 

Discovered double check - isn't this most often achieved via a knight discovering a rook?