In terms of max level characters:
BfA (60ish)
Legion (35ish)
WoD (28ish)
MoP (24ish)
Cata (9)
Wrath (3)
Not really related to how easy or hard it was to level, just a case of me already having more higher level characters and preferring to level new ones over paying absurd amounts of money for character transfers. I actively played the most characters during mid-late Legion, doing weekly keys on all 12 classes, doing every mage tower and getting every legendary every class.
In SL so far I have 12 and hopefully won't need to level more, but who knows.
In terms of leveling from scratch, probably MoP. After that a lot of the characters are the same just picked up and abandoned as needed.