First, I'd like to say that this information has came from the wowwiki website and pictures are from Thottbot.com. I just compiled it to this guide that I use when I leveled up my paladin, and 2 mages.
Level 21 to 25
Mosshide Gnolls in the Wetlands (see map below)
Code:
There is 1 caster, just polymorph him to pull that encampment, or kill him first.

Level 25 to 28
Dragonmaw Orcs in the Wetlands (see map below)
Code:
There are a few casters here, a healer is recommended but not needed. I multi-box and usually have my Pally pocket heal.

Level 28 to 31
Vile Fangs and Tained Ones in Duskwood (see map below)
Code:
Lots of vile fangs outside of cave with no ranged or casters. Excellent location for lvl 28 - 32. There is also camps over the hills to the left. 2 of these are full of dark runners and are very accessible

Level 31 to 34
Syndicate Humans in Alterac Mountains (see map below)
Code:
Cap with tents on a ridge, - watch out for patrolling cats and mages

Level 34 to 39
Various Beasts in Stranglethorn Vale (see map below)
Code:
Quest stacking fro the camp allows for some very fast experience

Level 39 to 41
Giant Buzzards in the Badlands (see map below)
Code:
Around 10 buzzards grouped near a carcass (visible on the map) just South of Kargath, approx co-ords 18,60

Level 41 to 45
Marsh Murloc in Swamp of Sorrows (see map below)
Code:
A good spot in my opinion. About 20 murlocs scattered in 3 spots really close to one another. Just be careful when using Frost Nova near one of the wandering crawlers, they tend to get on one's nerves. Allys also need to keep an eye out so they don't blink into the nearby Horde camp

Level 45 to 51
Southsea Pirates in Tanaris (see map belows)
Code:
3 clusters of 10 mobs inside the walled compound. There's one cluster of swashbucklers on the dock. Another cluster of dock workers is on the unfinished boat. The largest is the cluster around Andre Firebeard; beware of a patrol which is sometimes ranged. Kill the Freebooters separately before any pull (they are ranged). Each Pirate is worth 5 rep with Gadgetzan.

Level 51 to 55
Undead in Western Plaguelands (see map belows)

Level 55 to 58
Scarlet Lumberjack in Western Plaguelands (see map below
Code:
Watch out for the non-Lumberjack Scarlet mobs. Not the most choice area for Aoe because of Scarlet Knights and a few 58-59 mobs.

Level 58 to 60
Scarlet Worker in Western Plaguelands (see map below)

I hope that someone can get use out of it, it has gotten 3 of my toons up to 60 and my buddies' 2 toons up to 60. Enjoy!
Abaraikenshi