Interesting video on power leveling though a lot of it is kinda filler and thus it could be significantly shorter:

https://www.youtube.com/watch?v=uOzMu6PrChc

Assertion that XP is based on the average level of the party with no deduction based on having 1 level 60 in the party hitting gray mobs.

It denotes the following as the top ranked single level 60s to carry 4 low level alts:
Mage
Paladin
Warrior

Curious why a level 60 Warlock with hell fire or rain of fire & eye of killrog wouldn't also be good and if so where it should go in the ranking...