I grouped in Blackburrow and Antonica between 10 and 20 (you can do the spiders in BB around 18 or so). I also did Stormhold, which is good till you're in your mid 20's.
After 20, try to get a group in the Thundering Steppes, then around 25 or 26, move to Varsoon's or keep going in TS.
If you're on the Freeport side, then I'm not sure what to tell you.