Zygor is the premiere leveling add-on.

[Sad_Orangutan]

I love using Zygor when alt-leveling (going to come in extremely handy in the new upcoming expansion, too). My only criticize is that the customization options seem to have been slightly paired down more than previous iterations. Personally, I'd love to see more customization on the window itself- there used to be an option where you could choose square, or rounded corners (I realize it's a minor concern, but still a preference). Also, another quality-of-life feature I think would be a great addition is a transmog set guide- to where you can find guides on how to obtain certain sets of armor, and what to do to obtain them (and if they require currency, bullet-point suggestions on how to get them possibly?).