Depends on skill of player of course. Assuming very average skill with muscle memory binds. if you're good any class from D to A will work really well. You need to play with what you're good at. For example, a well played spriest will go toe to toe with boomkins for damage. They just have less intangibles.
Some classes like an UH DK will have a perfect blend of survivibility, burst and rot damage. Surv hunters can outburst an UH DK in melee attacks, but its a smidge lower on the list since it doesn't have all the other tools that a UH DK has.
THIS LIST DOES NOT REFLECT 1v1s! Blood DK and demo locks will win that battle (ipvp).
Subjective List for DPS CLASSES
*UPDATED 7.2
A LEVEL
Unholy DK -94
Fury -92
Balance -91 (nerfed!)
Demo -90 (insane tankiness #1, CC toolkit #1, rot dmg #3)
B LEVEL
Survival -89 (insane melee burst #1, low surv)
Frost DK -88
Elemental -87
Ret -84
Frost Mage -83 (CC toolkit #2)
Affliction -82/65 (insane rot dmg #1, NO BURST. Malefic Grasp Build -17pts)
C LEVEL
Arms -79++ (find a good healer will drastically improve performance)
Fire -79
Arcane -78 (insane ranged burst #1, low surv)
Feral -75
BM -74
MM -74
WW -70
D LEVEL
Enhance -68
Shadow -66 (insane rot dmg #2, low surv. SQUISHY)
Affliction -65 (Malefic Grasp Build)
Prot War -63 (very annoying to kill, a pest)
Assas/Combat -63 (highly dependent on individual skill)
F LEVEL
Sub -58
Destro -50 (54 if rex is playing. he's a solid 54 in all of his chars)
DH -12 (DO NOT PLAY)
UNKNOWN
Prot Pala -?
Blood DK -? *very tough to kill*
Guardian -?
Brewmaster -?lol