Ok folks,
I know there are a few of these out there, but this is the one I use.
In Blasted Lands there is a guy named Grol the Destroyer in Dreadmaul Hold to the North West. This guy is a Lv 58 Elite that can not die. He just stays at 1 hp forever.
Here is a link to his Wowhead profile: Grol the Destroyer - NPC - World of Warcraft
Anyway he is a great guy to level on, there is a few trash mobs in the room but anyone lv 60+ should have no problem.
I leveled most of my combat skills to 400 on this guy.
Note: DKs, if your blood spec, once he hits 1hp, you wont gain any life back from your attacks. Just a FYI for you.
Have Fun.