Dont belive their lies!
blizz claimed it early in legion or late in wod. but it was just some bullshit they said to please their customers which never got reflected in reality:
hunters: mm has been the best hunter specc until they gutted it in 7.15. now its the worst.
mages: fire up to 7.15, frost since
warriors: arms at least in early EN, fury at least since 7.15
rogues: outlaw for the first weeks of EN, assa ever since
warlocks:destruction in early en, then affl/demo started pulling ahead. since 7.2 its affli all the way
dks: uh up to 7.15 or mid en, not too sure; frost at least since 7.15
shamans: enh up to 7.15, ele + enh have been pretty close in 7.15, ele now in 7.2
their statement has been true for monk, priest and dh tho