I will ask different question: Why the search for the hardest conditioning exercise? Why e.g. not the search for the most effective one, which will bring the desired results?
Isn't it a fun question? The most effective one is a whole another question.
Being spent and getting punched in the face.. Nah, I'll stick to training without all that excitement, beside that, I can't afford to replace teeth.. Haha3 rounds of full contact MMA sparring. Not fun.
In this case, I would
- rephrase the question and use "test" instead of "exercise"
- and add an answer: 3 rounds of full contact MMA sparring. Not fun.
I've only almost puked once in my life and it was a long duration fin against a pretty fast current in a rebreather.