He'll obviously and deservedly be most remembered for his shows and his role in Elf for a different generation, but I always think of a random sketch he did on Mad TV that is among the very funniest things I've ever seen.
He was one of the best when it came to deadpan delivery. And instead of trying to overcome a stammer that some may have considered too big an impediment for such speaking roles, he managed to make it part of his persona.