4, 16, 32, 52, 86, 103, 128, 256, 1024.
When I mean by which would be the best, I mean which would make more money I.E Mine faster or find the block quicker.
Running at 128, I get 0.00228638 BTC every 24 Hours
Most pools will auto change your difficulty (VarDiff), but I would try out 256/1024 and see which runs better for you. Its a little bit of trial and error so give both a try.