Actually surprised nobody is saying Col. Jack O'niel... but I guess someone else is always imagined as that role.
He and John Carpenter have a great rapport, and it comes through in the movies. Outside of Tombstone, I think Carpenter's movies are the main ones where that comes through to enough of a degree that you just really love his character. He's been great in many movies, but there's a likable quality that comes through in every Carpenter film he's been in, and then in Tombstone.
:hmm: we need Dennis Dun (supporting actor/real hero in BTiLC, FYI) in more movies, too.
<- wore out his VHS of Stargate