Hmmm
Well, there is clearly going to be some variation in people's experiences. I already see 2 publishers listed here that I have had bad experiences with, and one more that I know has treated a few other people unfairly. I don't know how much help a thread like this is really going to be.
In any case, it should be obvious that I love working with Necromancer Games and do so almost exclusively lately.
I also love helping out on projects for Dark Quest Games. Neal Levin is a great guy and easy to work with.
The SSS/WW guys are cool and I would have no problem working them (basically, for some of the Necro projects I deal with these guys and I think they are great), and there are some others that I'd have no problems working with. Obviously, WotC and Paizo, although large, have a reputation for treating writers fairly.
Patrick