The age-old question is "to do what"?

I have a trio of Druids at the moment, paired up with a duo of Shamans driven by somebody else and I can't think of anything more fun for PvP.

Erm. Actually, I can't think of anything more effective for PvE either. They can all do pretty much everything to a greater or lesser extent.

In your shoes, I'd personally go for a pair of Boomkins - plenty of ranged firepower, adequate tanking, great HoTs, stealth, stealth, stealth...

But if I had to choose something from your list, I'd go with Mage + Lock.