Funny thing is, the Nicktoon that has the most movies is Fairly Oddparents, and yet none of them are theatrical. It's a real shame that series never got the Rugrats treatment. I don't think Channel Chasers or Abracatastrophy would have played well to a theatrical audience, though. I like the fact they have more dramatic moments than the show usually has.
Other than that, it's a real shame the only Nicktoons to get animated theatrical movies were just Spongebob, Arnold (and the wrong film to boot), Rugrats, and Wild Thornberries. Not counting Disney's Doug or that live action thing that was like Harmony Gold got a hold of Avatar. Also not counting Barnyard and Jimmy Neutron, since they debuted as movies. Besides... Barnyard was far better as a TV show.
Sigh... a Ren and Stimpy movie that would be a collaboration between John K and Ralph Bakshi... that would be something.