You know, I thought at first that this would be a really tough one, but when I actually thought about it, the answer came up pretty fast.
Taxi Driver is a great film, no doubt. Pushed some of the limits for violence at that time period, gave us the classic "you talkin' to me?" scene, so on and so forth. But I think when it boils down, the choice becomes Animal House. Despite only being 21 and not having been born when it came out (and by proxy, certainly not being their target audience when it came out anyway) I can still watch this movie and laugh at it, which is a VERY difficult thing for comedies to do. After a while, you get desensitized from their jokes, but I still find Animal House hilarious, while Taxi Driver has sort of lost its edge on me after about the third time of watching it. The last time I had it on, it was just on TV and I decided about midway through to change the channel as there wasn't anything to look forward to anymore. With all due respect to Taxi Driver, my vote will be going to...
Animal House.