I voted for the Spider-man franchise, as I think the movies so far have been absolutely spectacular and hopefully such high standards will continue with the third film next year.
I found X-Men The Last Stand somewhat of a letdown mainly because of the treatment of Cyclops, and the overusage of Wolverine. Both Superman and Batman had disappointing third and fourth installments to their franchises sadly.
Considering how Hulk and Hellboy only have had one film, both of which I enjoyed, I don't consider them to be so much of a franchise as the other films.