Just thought I'd throw in my 2cp, since I've found levelling to be pretty easy.
1-10: For my first 10 levels or so, I fought in and around Shar Vahl. There's a lot of newbie quests to be done here, and most of them are worth it too.
10-25: For about 15 levels (not as long as it sounds...) I fought in Paludal Caverns. I know a lot of people say this zone is horrible because of what it does to your skills by levelling you so fast. There is some truth in that, but as a beastlord you can solo a lot of the creatures in here, and work on your weapon and casting skills.
25-late 30s: After leaving PC, I visitted several different zones, most of which I don't really remember too well. I know I did some time in OT as well as a lot of time in Dawnshroud Peaks. Neither of these have experience modifiers, so everything felt slow after being in PC for 15 levels. This is one of the slow times for beastlords, and it certainly took it's toll of frustration on me.
39 - 49: I headed to Echo Caverns at this point and started killing underbulks. They are easy solos with your 39 pet, and decent experience. This is also about the time I got LDoN, and I wish I had picked it up sooner. The experience from adventures is /great/. I highly recommend it to anyone.
49-50s: Just dinged 51, so not much I can say here.... I'm still mostly getting experience in LDoN, though I've done a few trips into the planes as well. Experience seems pretty good in the planes, but I haven't had much luck getting a group there.
Hope this helps someone. My biggest suggestion to anyone over level 20 is to go do adventures. You get great experience, good loot, and practice in a group situation.