For the moment, I gotta go with DREAM. The Grand Prixs are great and add a level of excitement that has been sorely missed for a while. Their production is top notch, they have lots of well-established fighters, and Bas rockin' the commentary.
Affliction has some serious potential if they can lock up the big names for more than just a couple of fights along with stepping up their production value. EliteXC and StrikeForce are already seen as "the minors" and will likely remain that way to the casual fan. Remember: to the average North American, it's still "ultimate fighting" NOT MMA.
DREAM has the added advantage of not having to compete with the UFC on its own turf. It is essentially filling the void left behind by PRIDE's demise. DREAM has its own unique flavour and isn't simply trying to duplicate the UFC product and put its own stamp on it as the North American promotions seem to be doing.
Maybe it's just personal bias speaking, because I really, really like DREAM. Is it too soon to call it love?
I'm considering seeking out expert opinion on the matter.