For me, it has to be this... YouTube - Nintendo NES Australian Commercial #1 It's just so freakin' terrifying!
Great topic! There's so many to choose from really. The first Mortal Kombat launch for consoles, the recent Halo 3 campaign, too many to choose from. But my hands down winner has to be the Japanese Chu Chu Rocket commercial for Dreamcast. YouTube - Chu-Chu Rocket Commercial
i quite liked that one for mercenaries 2, with the "oh no you didn't music" never played it though. *searches youtube* found it!
How about a little Ice Hockey with Phil Hartman? YouTube - Phil Hartman Wigs Out for Hockey for Atari 2600