If we're talking about the movies, it's still hard to pin one down. I think it's really between the Aliens and the Terminators, though. I LOVE the first Predator movie and like Predator 2 a lot as well. Heck, I even like both AVP movies, but none of those are as good as the better Terminator and Aliens movies. Predator is a classic action/sci-fi flick, but when compared to Alien, Aliens, Terminator and T2: Judgment Day, it's just not quite up to par (though it's certainly better than Alien 3, Resurrection or T3).
The Aliens franchise was marred by Alien 3 and Resurrection. Like I said before, I actually liked the AVP movies but I acknowledge that they're not great films. T3 is not a great film either, but the new Terminator: The Sarah Connor Chronicles is quite well done. I can't really take the upcoming Terminator: Salvation movie into consideration, but it's hard not to, with Christian Bale as the lead. I think that, overall, Terminator has been the most consistently good franchise, whereas there has been quite a few disappointments with both Aliens and Predator.
As a toy line, it's gotta be between those two, though. I don't own any of the 12" Predators, but they do look awesome and as soon as a P1 version comes out, it's going on preorder. Same with Dutch (and maybe Billy, as long as both don't end up looking like Kane from Alien). The Aliens line is great too - I only own a Drake (at the very least, I must have Vasquez to go with him - please help me if you're looking to sell!) in the 12" line but I've got all the 1/18 scale snap-kits (except AVP series 1, which I desperately want as well) and the Queen, which are really great. Then you've got all the statues, Medicom stuff, Takara Microman stuff, etc... I'm thinking Aliens might take it, toy-wise, but it's close.