Honestly there are a lot of good zones to level up in, but realistically (at least on my server) the only zones you can get groups in, with plenty of people around, are the Luclin zones for the most part. PC, Dawnshroud, etc etc. Of course, if you can get a group together, you can go to any of the zones mentioned above and get good exp. I did LOIO from 16 to mid 20's, FM from mid 20's to 30, and OT from 30 to about 38 or so. But not many people in those zones now.
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Some people are like slinkies, they aren't really good for anything, but they still bring a smile to your face when you push them down the stairs.