As Amastropolo said, Hot Zones are a good way to go. LDoNs are still good xp, and the Adventurer's Stone is still one of the best charm augs in the game when maxed.
If gear is important, the TSS expansion has a full set of zones to level from 1 to 75 in, and lots of quests for all types of gear. But if platinum is no object and you want to level fast, there are many better zones for experience. TSS zones are not bad experience but not the best especially at lower levels, and the mobs tend to be tougher there than other places as they get higher level.
Monster missions never gave me that much regular experience for lower level toons, but I see alot of people that do them to level. Basically, imo, they're only good if you don't have even cheap bazaar level gear, since you don't need any to do them. However, there are some nice loots that come from particular ones, and I've heard Dain series is good experience, but I don't have PoR, so haven't tried it out.
Yther Ore.
Edited, Jun 18th 2007 2:58pm by Yther