You can make workouts - cardio or weightlifting - ridiculously intense just by timing them, as that forces the person to go as hard as they possibly can. So for me, it's a toss up. Crossfit uses a stopwatch in almost all of its "metcon" (metabolic conditioning) workouts, so a lot of them are absolutely brutal. Some of the most mentally and physically demanding workouts I've done:
* All out 400m run. Has the lovely distinction of seriously taxing all the energy pathways from phosphagen to glycolytic to aerobic as the run goes on. 800m and the mile are not far behind, but nothing leaves me quite as floored as a balls-to-the-wall 400.
* 20 rep squats. The only one in this list that isn't timed, but also the only workout I absolutely
dreaded. Worked my way up too 300x20, which was the single hardest set of anything I've ever done. After 15 reps, as you stand shaking and gasping under the weight, thoughts of impending death are common.
*
"Eva": lots of 800m runs and a ridiculous number of heavy KB swings and pull-ups. This workout always made me want to quit.
*
"Filthy Fifty": this chipper has 500 reps, the last 150 being particularly soul-crushing: 50 wall balls, 50 burpees and 50 double-unders.
* An
unnamed workout that included 125 burpees, 125 pull-ups and 125
inverted burpees (a kip-up to handstand). Again, the word "quit" always comes to mind.
*
Sprints: sixteen 100m sprints, with 45 seconds rest between each one. 10 burpee penalty each time you ran a sprint too slowly. I ended up doing 1 mile of sprinting followed immediately by 50 burpees. Absolutely everything hurt when I was done.
I'm sure there have been others that I've blocked out of my mind as well
🙂