I like the following:
1) From the episode when they are playing with the Japanese exchange student. Beavis hits a mud ball with a rock in it. It knocks Anderson out. The boys approach Anderson, who has a flash back to WWII. He sees the Japanese student and says: "Don't shoot me!" Butt-Head says: "Don't worry, dude. We're not going to shoot you." To which Beavis adds:
***** "Not today." *****
2) From the episode in which they forget how to urinate. The boys list terms for urination. Butt-Head says:
***** "Uuuuuuuuuuurinate."
/rtm