My opinion on what might be the best synergy:

Druid - Feral
Shaman - Elemental
Priest - Holy
Locks - Affliction or Destruction for both.

Simply put - I think the Holy Priest is a bit more flexible for group healing. That forces the remaining casters to be DPS.

You need to explore the synergy between the Shaman and Lock casting styles if you go this route. The more you homogenize the casting sequences, the less complex your encounters will be when you start dungeons/raids. I try to min/max my teams as much as possible, including what spells each character uses. Locks have cast times and cooldowns that are not like the other caster classes and I've tinkered with integrating them into my teams in various ways. I've personally found that destruction works best for my playstyle. For example, Mages and Balance Druids match up nicely with cast times & cooldowns: wrath + scorch, fireball + starfire. Once my Balance druids are higher level, I pair them up with a Locks opening dot sequence (faerie fire, insect swarm, moonfire w/ Curse of Elements, Corruption, etc.) and then use a fireball + starfire + shadowbolt setup. Sometimes I change to wrath + scorch + shadowbolt.

I also watch the synergy of mana costs very closely. I don't like to stop the whole team for one character to eat or drink (just a personal pet peeve) and I pay close attention to talents that affect cast times and/or cooldowns. Sometimes you want them, sometimes you don't depending on the overall team cast synergy.