Which one was the largest?
We gauge the diffrence between best pre-raid gear you can get before stepping into the first raid of an expansion and maxed out character in the last tier.
I think it was in WOD, even though this expansion had only 1 major content patch and didn't really had any aux systems to ramp your power through the roof.
I was doing about 20k single target on heroic Butcher which was typical DPS check fight, about the same on Imperator Mar'Gok heroic (but that fight was more mechanically intensive AND i had some gear off other bosses and bonus rolls already even tho we killed that boss in week 1). I had very good gear coming into raids, bought the best BOE's that were available.
And at the end of WOD I was routinely pulling 200k, although in a very short fight (such as killing Iron Reaver before he even goes up into the air). That was like 10 times more damage than when starting out. Almost no systems. Just excellent tier set bonuses, high item level (augmented by valor point upgrades later in the patch) and the legendary ring. The ring was the only "system".
Idk about BFA because I skipped most of it. I was doing about 110k DPS at the end of it, with maxed corruptions etc. Maybe it was even bigger power gap.
It seems that 5X power gains are common since Mists of Pandaria. It's not just systems thing. It's high item level gain mostly.