Planet of the Apes.
Admittedly, I didn't see it until after seeing Rise of the Planet of the Apes, which is far superior and kind of gives away Planet of the Apes's "twist" ending, but I found Planet of the Apes to be lacking, partly because the only thing there was to the ending was "OMG I've been on Earth this whole time!" Which, if you really think about it, makes the most sense anyway; what chance is there that you're going to run into extraterrestrial life that's exactly the same as life on Earth with only one minor difference, and even speaks the same language? It's just as likely for another Statue of Liberty to exist on another planet.
Other than that, I just found Planet of the Apes to be filled with stupid. I mean, I don't care how primitive the apes are, why in the holy hell would they not leave any recordings at all of their great victory over man? Why would they completely abandon human technology which they are perfectly capable of using and start from the stone age? Why didn't they even keep records of this technology (evidenced by one of the apes claiming that flight is a "scientific impossibility")? And if they did start from the stone age because they're too stupid to adapt human technology for their own use, how did they manage to permanently make all humans so stupid that they can't even talk, a feat which would require a modification of DNA that even we, with modern science at our disposal, cannot currently make (AFAIK)? The whole movie was loaded with questions like that.