I’d say Robin Hood: Men in Tights and Hot Shots. (Hot Shots Part Deux wasn’t bad either: “War- it’s fan-tastic!”)
I’d say top honours to Spinal Tap, for stunning attention to detail, characterisation, gags and accuracy.
A close second to Top Secret, which got much less attention than Airplane but which I felt was much more inventive and ingenious (e.g. the entire scene that was filmed backwards, but this isn’t apparent at first).
I don’t have a favorite, because I like spoof’s so much, but I can’t belive no one has mentioned National Lampoon’s: Loaded Weapon 1 yet!
Mortician: It’s an apparent suicide…
Sgt. Luger: Looks like she caught herself by surprise.
Sgt. Luger: Oh yeah? Well these matches say you’re lying!
[hands matchbook to Squealer, he opens them up and inside is printed: “You’re Lying!!!”]
The Naked Gun has some classics too…
Frank Drebin: [seemingly looking upskirt] Niiiice Beaver!
Jane: Thanks, I just had it stuffed [hands down stuffed beaver]
Hey, I mentioned Loaded Weapon I!
By the way, you can buy it on DVD at Amazon for $13.48. But why would you want to do that? Most places sell it for $10.
Never thought Spaceballs was funny.
Spinal Tap is pure genius and Naked Gun is a classic. Just watched it on Comedy Central with Leslie Nielsen signing the National Anthem. Total crack-up.
Sheesh, did no one else here love Rat Race?
Okay, I have to confess that while I did love it, Airplane and Young Frankenstein are probably my top two. But plenty of people here gave a list, and on mine, Rat Race is # 3.
I don’t think I ever laughed as hard as when I watched the scene where Cuba Gooding Jr, driving a bus full of Lucille Ball impersonators, watches a cow hit his windshield as if it were a mosquito. 
Spaceballs is definitely the best spoof. Mel Brooks is the spoof master!
“YOU IDIOTS! This isn’t them! You’ve captured their stunt-doubles!!!”
Well, it’s only a short film (9 minutes), but it deserves a mention: George Lucas in Love, a hilarious spoof of Shakespeare in Love and Star Wars. Seek this one out if you haven’t seen it.
oops! sorry Mofo, my bad 
guy in stadium: He SAVED the queen!
guy #2: [points excitedly at Frank Drebin] It’s Enrico Pallatzzo!