While I chose 1-10 on the poll, I actually like the whole 1-20 stretch. Of course I have my preferences on zone for these levels as well since I'm not fond of some of the starting areas. It always seems like you are making great progress because these levels don't last long and you are constantly gaining new and important skills. I know the quests so well that I can speed through them fairly efficiently which also helps keep my interest high.
21-30 is a blast, particularly thanks to Duskwood. I despise 21-30 on Horde, however, so most of my characters are still stuck there.
i say 31-40 because of SM i just loved that whole scene (i level by doing instances and am known as a some-what reliable but a good dps hunter in my realm) neways i just loved learning about that whole place and its lore and history. i must have done each instance almost a hundred times now (on each character) and just love its ability to teach about proper grouping roles(next to ZF)
I like 40-50 as well; getting a mount is a relief, as is getting out of STV. I like Feralas, and Tanaris is fine.
The worst bracket is 40-50, there is little to shoot for during that bracket and the levels seems to just crawl by. The 50-60 bracket isn't bad because you are looking forward to outlands/epic mount and 61-70 was a blast because of the variation.