2x Shaman - Both can self rez and rez each other. Both get ghost wolf for increased movement. You can offset their shock spells to constantly have an interrupt available.

2x Druids - Both can rez the other, both get cool animal forms for more diversity. I am currently leveling 2 druids as casters, but when I come across something I dont want to have to fight my way thru I just go cat on both and stealth thru with no problems. Add the insta travel form for quick getaways, bear for bullying thru if you have to, aquatic for underwater breathing/faster swim speed, and of course bird(flight) form at later levels and I think the druids are awesome.

ShadowPriest/Lock - Great synergy between the two. SS the priest so that if you wipe for some reason you can recover easier. You will have little downtime with this pair. The priests ability to regen mana while casting(talent spec) and the locks ability to lifetap means less stopping to drink.

Mages are awesome with just about any other caster type. The dps you will unleash together will pretty much stop anything in its tracks. Add the abilities of the mage to summon water/food, and portals back to the main cities it will make any combo much easier in the long run.