I've tried all three. Extract gives you the best chocolate flavor without all the extra hassle.
By hassle, I mean trub, secondary additions, figuring out how to sanitize, etc.
The nibs are also a very good option, but to me they give off more of a bitter chocolate flavor. I typically use powder because it's easy to come by and gives a nice subtle flavor. I went through a chocolate stout phase, but to be honest, I'm sick of them now. When I use chocolate, I don't want it to be a strong flavor.
If you want over-the-top flavor, then use nibs or powder late in the boil, and add extract at bottling time.