...and why.
Let's see a list of the places you like to level and the reasons why.
Make sure to let us know if you are coming from a horde or alliance perspective as different areas obviously offer different things. Also let us know why you like this spot. Quality over quantity here, better to list one zone with a good backing then to list 20 zones with no comments.
I'll go first.
Alliance
Level 11-17ish
Westfall - I never get tired of showing the defias brotherhood who is boss. The questline that ends with killing Van Cleef (and then continues on to the stockades and beyond) is one of my favorites so far in the game. It really feels like each step is building on the previous encounter from dealing with the lower level thugs to tracking down the hideout in Moonbrook.
Horde
Level 19-21
Wailing Caverns - I usually hit this instance with my toons right at the start of 19 with all quests ready to go (all 5 of them, plus the one inside!) and walk away from the experience with usually a few good items and halfway to level 21. I really enjoy this zone even as long as it is. The bosses though not necessarily challenging are fit to the theme of the zone. Verdan the Everliving, that humongous walking pile of leaves always inspires just a bit of awe in me as I fight him.
Okay, so there are some examples, let's hear yours!