Definitely Spiderman.
Both Spiderman movies (especially the first one) was mainstream ...in that it was enjoyed by not only comicbook fans but the general populace as well (and its box office proved this in a quantitative manner).
The X-men movies on the other hand were mostly catering to comicbook fans. Both were good movies, but definitely not at the same level as the SPiderman movies (again, particularly the first Spiderman movie).
Anyways, I'm really REALLY looking forward for the Spiderman 3 movie ...not only because of Venom, not only because I like Spiderman - in comics I actually prefer the Xmen -, but because I know it will be a good movie both in terms of PLOT (something most movies today forget to include and just have explosions and stuff) and visuals.
I know I'll watch the X-men 3 movie, but I am definitely not on the edge of my seat for it. I'm definitely more excited over the new Superman movie.
Anyways, for ME I'd have to say Spiderman 3.