Note: This guide was written during 2008 when Underfoot was the main expansion pack. It may be inaccurate for players currently on live. It contains a lot of information, some of which may still be useful to players which is why I have decided to preserve the guide. If you haven't already I recommend you read my Live Servers Crypt of Decay Leveling Guide. Those of you who play on a TLP server will want to use my PoP TLP Crypt of Decay Leveling Guide.
One of the best zones to level in through the 60s is Crypt of Decay. This zone has tons of mobs in it of all kinds. Upon zoning in the first type of mob you will come across are the Undead Puslings. These guys used to be strictly Undead which made them extremely fun to lay waste to on Paladins but recently they were changed to be Humanoids as well which makes them Headshotable by Rangers.
Further north upon entering the zone you will come across the Bubonian Rats which are all Humanoids and for the longest time were camping grounds of Rangers with the Headshot AA looking for some quick XP. Sometimes you will still catch a Ranger here but since Headshot got some horrible nerfs it's increasingly rare to see a Ranger using it to grind out some XP.
Regardless, you can take out the Puslings or Rats if you're looking for some XP, I recommend skipping the mobs that spawn around Carprin to the west just because it seems that Carprin is always alive and here to kill people. And trust me at the levels you'll get XP here taking him out won't be an easy task.
Whichever you decide to kill in Crypt of Decay you won't be dissapointed. The loot is excellent here and the mobs are fairly easy to kill for very good XP. Be sure to hang on to all the Cultural Items you get to sell in The Bazaar as well as the scrolls which are used in Spell Research and sell for a ton depending on what server you're on.