Not sure which I'll do first, as I like all three:
- 5x Feral Druids
- 1x Holy Paladin & 4x Frost DK's (experimenting with Unholy too)
- 5x Holy/Ret Paladins

I've always liked 5x Shaman.
When I was really into PvE (mostly heroics), my team was Prot Paladin, 2x Elem. Shaman, 1x Elem./Resto Shaman, and 1x Demo Warlock (my raid toon).
My current version of this team is 1x Guardian/Boomkin Druid & 4x Elemental Shaman.

I like my Warlock team (1x Priest, 4x Warlocks); they're Cenarius Horde.

The Pally/DK's and Pally teams have been transferred to PvP Mag/Horde, and are in the The Zerg.
The others are still on my original server, PvE Cenarius/Ally.

I'm tempted to make the leap to 10-boxing, having gone as far as picking up the Classic-Cataclysm accounts... when they were $20 each.
That will be another set of 5x RAF accounts.
I upgraded my system over the summer (Sabertooth X79, i7 3930, GTX 680 4GB, 32GB Ram, 2x OCZ 3's in Raid0), both in anticipation of higher game requirements and knowing 10-boxing is a temptation.
I'm leaning towards merging the teams... 6x Paladins, 4x DK's... in some combination of Holy/Ret and 3x Frost/Unholy with 1x Blood.
10x Ferals will be fun too.