I personally prefer to level in TreeSpeak, but I am a Twin Adder, so I burn 4 Guild Leves, then perform a few company leves, and throw in some Battle warden runs in (save the battle warden for ~43 when soloing). I normally would do 4, 4 and 2 (without the battle warden, just stick with the 4 and 4 at whichever company you belong to). Be sure to set the nearest camp as your favorite, and your home point in that city.
After you finish, return, flag 4 more, and repeat. You should be able to average around a level a run, and each run is about 1-2 hours, so not too bad (around 7-10k exp a run).
This, pretty much. Treespeak got the best 40 leves IMO, and if you're a Twin Adder you can also do company leves. If you can't, it might be a bit harder but it's still quick enough to set treespeak as home point and port to grid / return when you run out.
Just a suggestion if you're doing company leves for levels: Keep doing them till you get an escort mission, then do a regular guildleve to switch the current company leve they're offering; escort missions give crappy exp and are just a waste of time. Also, save up Guardian Aspect for the kill em all leves, and always have the monsters in those leves in a higher level than you: You're gonna get 10K+ exp just on that single quest that way. That can be done regardless of where you're doing leves, twin adder or not, and is equally good advice for the level 30 company leves. Edited, Feb 10th 2012 6:25am by AdvancedWind