"What's the best prime on a crop?" If you figure it out, let me know. I have been tearing my hair out trying to choose which prime is next for me. I already have the 60mm, so the nifty is not a likely contender; the 60 shares some of the same benefits as the nifty: small, light, super sharp.
I want a very shallow Depth of field, I don't tend to shoot really wide too often, and I already have a really good Ultra wide zoom, so I have been thinking a little longer. It will probably be one of these: 50mm f/1.2, 85mm f/1.2 or 135mm f/2.0. But they are all pretty expensive, so it will be a while.
However, I also highly recommend the 17-55 as a really good all around lens for a crop! Parked in the middle of an Ultra Wide and a 70-200, it makes for a really nice kit.