TwoSix
Everyone's literal second-favorite poster
I'd really want to see the calculations and assumptions on that before I'd agree. 6d12 superiority dice is 39 average damage per short rest. Baseline, an 18-20 crit range would add 0.83 damage per attack, assuming a 2d6 weapon, GWF, and no other bonus dice. That would need 48 attacks per short rest to catch up, which is 12 rounds of 3 attacks plus either a bonus action or reaction attack per round.However, I disagree that Champion isn't a good choice. Fighter is the tops of the melee class damage due to Extra Attack (2) and (3), but last I knew, Champion was still the king of expected damage per round simply due to the expanded critical range. Six d12 superiority dice don't keep up after the second round (or the first if you action surge). Champions are relatively boring to play, and I would personally never play one without access to feats, but mathematically they're the best at damage and Survivor (regen hp up to half max) is stupid good.
If there are any other bonus dice involved, or you assume the BM fighter is using the dice for anything besides damage, I could see Champion catching up, yes.