What is the worst opening move, and why? Grouping of initial moves of a chess game are well known, though I've never seen analysis of the worst possible opening move, any thoughts? If so, please provide more than just a move, but the exact reason why such a move would be the worst assuming such a move was against a GM.
1. f3 is almost certainly the single worst move.  It irrevocably weakens white's king position without doing anything useful.  

Other moves like 1. b3 and 1. g3 prepare to develop a bishop, so they're not so bad.  1. c3 doesn't do too much for white, but it also doesn't hurt him.

1. a4 and 1. h4 both ignore the center, but they do help develop the rooks, and there are positions where 1. a4/h4 can be useful (especially when the kings have castled on opposite sides of the board.