The Black Knight is funny, but after hearing it quoted approximately eight billion times, I've tired of it. The Knights Who Say Ni were never particularly funny. Castle Anthrax is a great bit, but not quite as great as this...
Monty Python and the Holy Grail - Lancelot Storms the Wedding
Everything about this scene is great absurdist comedy. My favorite part is right at the beginning, where the guards watch Lancelot running at top speed waaaay off in the distance over that same little stretch of ground multiple times until right as you're about ready to write it off, then - BOOM - stabbings!
Lancelot then goes on a rampage, murdering pretty much everyone in his path. It's as psychotic as it is hilarious. Once he gets to the top of the tower, it's just some ninny, and Cleese's reaction is fairly priceless there, too.
Funny fact, this was the last movie I ever watched on VHS. My brother and I had nearly worn this spot out by then. The part where Lancelot stabs that first guard has gotten gotten the biggest laugh in the entire movie.
Let's relive it.