I'd go 55-60 in Ramparts.

With a moderately geared Prot Paladin (mix of heroics, badges, ulduar 25 and ToC AH stuff), I boosted 3 teams for my roommate in there, 55-60 (RAF) in a day.

Just kind of ran along killing things, with him looting behind me.

At 60th, move to Underbog.