It's not particularly important. Doesn't lie close to the center, no important diagonals, nothing. It's just a square.
h4

bomtrown> ok, what if you were a bishop on h4 and wanted to get to d2?
Without looking at a board, h4-e1-d2 or h4-g5-d2. In a real game, the dark-squared bishop usually goes to the h4 square (from c1) after Bg5 h6 Bh4.


what would be the least used square in chess?
Don't know. But I'd say it probably depends on the game. In this diagram, we see all of the various sub-boards that make up the 8x8 chess board. It is easy to see that the corner squares are most isolated, but that doesn't mean that they are least used.
So uh, what do you think of the square h4?