1) Unholy Death Knight
2) Affliction Warlock
3) Sub Rogue
I picked Death Knights as the #1 class simply because they can be paired with so many other classes and create a devastating combination. Their survivability is absurd due to their numerous cooldowns such as AMS and Lichborne.
Affliction Warlock was my #2 because they pair so well with Death Knights but also do well with multiple other top classes such as Sub Rogues, Resto Shamans, and Resto Druids. Their ability to completely tank casters & large amount of instant casts really makes them tough for many of the top comps.
Sub Rogue was my #3 for the same reason Death Knights & Affliction Warlocks were my #1 & #2. Their synergy with Affliction Warlocks & Resto Shamans creates an insane combination. The amount of control on enemy casters & pressure on enemy healers is some of the best in the game.
1) Resto Druid
2) Resto Shaman
3) Disc Priest
Really not going to write a description as elaborate as the ones above simply because I'm pretty sure I covered most of these classes above. I put Druids above Shamans simply because of their CC. Priests are above paladins because paladins are so insanely vulnerable to CC where as druids are protected by hots, shamans have plenty of ways to stop it, priests have bubbles & prayer of mending.
Also, for those who want to know. Frost Mages didn't make the top 3 because of their vulnerability to being tunneled & the fact that every healer now has a defensive magical dispel. 1.7 second polymorphs that get dispelled in a global (assuming you get it off w/ the enhance shamans & feral druid on you) is pretty fun. Also, their burst damage is pretty laughable now (with the exception of the occasional Brain Freeze proc). Also, 2 second cast Frost Bolts that crit for 13.5k on frozen targets (after 15% buff) is also pretty threatening.