Supposedly, Ret is the best spec to level with if you want to get her to 80 and quick.
But, I wouldn't know myself. Levelled from 1-67 as Prot, was my first real class I pushed past 50, and then dual-specced Holy for PvP. In the end, I fell in love with Holy, and levelled the rest with that.
As Prot, it was more effecient to take on whole camps at a time, popping the somewhat larger CDs like Bubble and what not, but downtime wasn't an issue and it was effecient. Also, incredibly fun. I found Holy to be nearly similar, but instead of DPSing the mobs down before you die, I kept Consecration and Retri Aura up whilst healing myself through the good 20+ mobs tailing me.
All in all, I'd say look through a talent tree that sounds like something you'd enjoy. Play as it, and re-spec' a few times as you level until you find something that suits you. Ret may be the best spec' for levelling efficient, but may not be the most fun to play in your case.