Oh man, I loved that movie. It's the sort of movie they don't make much any more because it's not a CGI film and it won't win any Oscars.
As it happens, I saw the original show in reruns, and it's all seventies action set-pieces, nowhere near as funny.
The two leads have great onscreen chemistry, but from what I remember, most of the laughs come courtesy of Ben Stiller. In reverse order, my favorite laughs were:-
(5) Ben Stiller shoots a lizard's tail off
(4) Ben Stiller questions a naked woman
(3) Ben Stiller shoots a pony at a party
(2) Ben Stiller dance competition, on coke
(1) A little kid throws knives at Ben Stiller
It was the last one that cracked me up the most. I decided if I ever make a movie, there absolutely must be a knife-throwing kid in in it! :)