I've always really liked Outland, maybe because I started in TBC, so getting to Outland was the beginning of the end so to speak. I think it gets worse (levelingwise) with each expansion since it's more XP. WotLK is alright, Cata starts to get slow and MoP is really slow. I've gotten two chars to 90 and enjoyed leveling to 90 the first time, but doing the same quests on several chars can get really annoying, especially when it takes ~4 hours (maybe) to get a level by questing loads.