Even the priests I have no intention on taking into dungeons remain steadfastly discipline-specced. Two of them have secondary disc PvP specs, to be sure, and a former main still shows the remnants of a secondary shadow spec -- but every time I've ever started to play her with that setup, I end up impatiently logging over to one of the mages for some real blasting. And no matter how highly the straightforward capabilities of the combat rogue are recommended, I refuse to veer from subtlety. Lurking in the shadows is way too much fun.
Besides players who are bringing up new tanks or healers inside the hallowed halls of instances, it's probably fair to say that most people consider the spec with the highest front-loaded DPS to be the right spec for leveling. They're rushing up a new character to the endgame, or they really dig spraying massive clouds of floating combat text. It's also probably fair to say that the old standard choices might be becoming somewhat less so, thanks to the recent talent system overhaul. Still, the "best" spec for me is neither the most efficient or effective spec -- it's the one that just clicks with me. This is true even for characters without a roleplaying bone in their virtual bodies.
What makes a leveling spec right for you? Do you look for the highest DPS for fast, efficient killing? Do you prefer a tank or healer, to guarantee quick instance queues? Or like me, is there simply a flavor you like best in your WoW bowl?