I'm gonna have to go with CJ. Even though they **** me off on a regular basis, I can't set aside the fact that they provide me with better tools for earning - best interface of the big networks by far, better linking options (product page definitely), best variety of good-quality affiliate programs.
I think SAS will be my favorite as they evolve. Right now, it's frustrating trying to find a good affiliate program because the search sucks and they're loaded with crap programs to sift through.
Linkshare has some good programs but the interface is absolutely horrid, as are their linking options (C'mon guys, nobody uses banners anymore!). And they simply don't seem to respond to all the feedback out there about these issues, which really says something about the company.
I do like AzoogleAds for my sites which are appropriate for CPA, I simply don't have as many of those as product-focused sites.
My .02
