With RAF...
And boosting with a 70th of the same class as each set.
Late Burning Crusade expansion.

Got to 60th:
- 5x Druid
- 5x Hunter
- 5x Priest
- 5x Shaman

Had a month of about a month of RAF left, but wanted Shammies to 70th in preparation for WotLK launch.
Have since leveled these teams:
- 5x Death Knights (85th)
- 5x Paladins (57th)
- 4x Warlocks + 1x Priest (22nd)

The more combinations you do the better.
And if you're boosting with a tank type or at least an AoE type that will help.
I'd imagine any 80/85 can solo level 60 instances, but a Pally with Consecrate or DK with Death & Decay/Blood Boil would be more efficient.

The RAF set I did instances.
The post-RAF was straight questing.

You could run instances, if you have a tank + healer + 3x dps.
Or even tank + 4x dps that can heal.