Is boxing the toughest mentally and physically?

Not when you're in the death zone (above 8,000m) - you have two choices... get out of there as quickly as possible... or die. Make a serious error? You'll be left there to die... no towel... no ref... just death. Even your closest friend will leave you to die up there.

Next to that, I don't think many sports can compete - but I'd put boxing up there at 2 or 3 with cycling (even with the drugs, climbs like Mont Ventoux or Alp d'Huez are shocking).

 

MMA wins.

Its everything you can say about boxing but requires more versatile training and fight plan.

They don't get the luxuries that pro boxers get.. The gold medalist treatment, different promoters, belts, surplus of weight classes, and being able to remain a world champion for years within a weight division and not actually being forced to give up your belt or fight for others. Pro boxers nowadays get away with too much so on a professional level MMA is a more demanding sport beyond the physical part.

You couldn't say half of this stuff about boxing twenty years ago.

 

You have never climbed an offwidth I see.