Space Mountain, I've rode it a million times and it NEVER gets old.
Second is EE, but ONLY in the front seat. The front seat line usually isn't that long (sometimes its nonexistent) and even if you have to wait behind a few people it's soooooo worth the wait.
Third, TOT, as another poster said, it's the thrill of not knowing, am I going up? Am I going down? When's it gonna happen? The rush from the anticipation never goes away.