Hollywood Naitch
The current reigning and defending
Guerrero isn't really that long as he won four years after debuting, not counting about a year out of the company
Just WWE Title:
Andre (difficult to say, at least ten years)
Sheik (around at least three years)
Slaughter (debuted in the early 80s but was in the AWA for most of the decade)
Bret Hart (6 years)
HBK (8 years)
Sid (4 years, wasn't there the whole time)
HHH (4 years)
JBL (9 years)
Edge (8 years)
RVD (had a few matches back in 1997)
Miz (4 years)
Then there's the mother of them all: Vince, who was on TV for about twenty years before winning the belt.
Wouldn't Jeff Hardy be on that list too? I know he had a couple of years out of WWE, but he was with them in 1998 or so and it was late 2000s when he finally won the big one?