I grabbed some data from Warcraft Logs trying to gauge the popularity of certain classes in Heroic raids based on the number of parses. Keep in mind the data set is limited to Warcraft Logs.
This is based on the 75-percentile statistics for Heroic Emerald Nightmare raids from today, 2016-09-25.
Class Popularity (based on number of parses)
Class Parse Count % Druid 43255 13.20% Mage 33768 10.30% Hunter 33331 10.17% Shaman 32993 10.07% Priest 30151 9.20% Paladin 28402 8.66% Demon Hunter 24425 7.45% Warrior 23119 7.05% Death Knight 22156 6.76% Rogue 21039 6.42% Warlock 19539 5.96% Monk 15620 4.77%
Class Popularity for DPS
Class Parse Count % Mage 33768 14.72% Hunter 33331 14.53% Rogue 21039 9.17% Demon Hunter 20058 8.74% Druid 20014 8.72% Warlock 19539 8.52% Death Knight 17289 7.54% Shaman 16862 7.35% Priest 15792 6.88% Warrior 15728 6.68% Paladin 8410 3.67% Monk 7561 3.30%
RDPS Popularity
Class Spec Parses Mage Fire 33,000 Hunter Marksmanship 25,026 Priest Shadow 15,792 Druid Balance 14,323 Warlock Destruction 13,427 Hunter Beast Mastery 7,834 Shaman Elemental 6,682 Warlock Demonology 4,981 Warlock Affliction 1,131 Mage Arcane 627 Mage Frost 141
MDPS Popularity
Class Spec Parses Demon Hunter Havoc 20,058 Rogue Outlaw 17,513 Death Knight Unholy 16,426 Warrior Arms 11,070 Shaman Enhancement 10,180 Paladin Retribution 8,410 Monk Windwalker 7,561 Druid Feral 5,691 Warrior Fury 4,658 Rogue Assassination 3,176 Death Knight Frost 863 Hunter Survival 471 Rogue Subtlety 350
Healer Popularity
Class Spec Parses Druid Restoration 17,815 Shaman Restoration 16,131 Paladin Holy 14,790 Priest Holy 11,274 Monk Mistweaver 7,113 Priest Discipline 3,085
Tank Popularity
Class Spec Parses Warrior Protection 7,391 Druid Guardian 5,426 Paladin Protection 5,202 Death Knight Blood 4,867 Demon Hunter Vengeance 4,367 Monk Brewmaster 946