I instantly thought of the Zs, but on thought, it probably is the square. Such a nice regular shape, yet there never seems to be room for it. I think it's a side effect of compensating for the Z shaped pieces though. You always leave one square gaps to fit a Z into, then a square comes along and doesn't fit.
But, if the whole game was just squares, it wouldn't be at all annoying. If the game was just Zs and backwards Zs, it would be a nightmare.
So I think I'm going to have to stick to the Zs (don't care which type). Even if you think you have a space for a Z, it will always be the wrong type and won't rotate right to fit in.