Two's complement 8-bit subtractor?
It may seem at first as though computing the two's complement of b would cost you a whole separate incrementer, but you can easily merge it into the adder that you need anyway: just set the carry-in.

Alternatively (for example if you are somehow forced to use an adder without carry-in, or if it just works out better in combination with other logic) you may used that a - b = ~(~a + b), which obviously only ever needs inverters and a normal adder. This does put inverters before and after though, whereas to calculate the two's complement of b you'd only need inverters before the adder (and a carry-in but that's almost free).