Alright... Starting from the beginning:
Season 3's Leg 12 featured a task that caused Flo to go insane, and just looking at those coconuts on the bikes just seemed way too exhausting, as did the boat.
Season 4's Leg 10 featured a swimming challenge where they had to swim underwater in frigid ice water. What more is there to say?
Season 5's Leg 4 in Russia gave us an example of helpless teams get overtaken by Chip & Kim because of fish eggs. They all looked genuinely tired and sick.
Season 6's Leg 3 gave us a completely hard leg with a crazy Detour where your head had to be in the game. The counting was just as much of a killer task as building the furniture, well.. That's debatable. Counting 2304 items is no joke! The Roadblock was so difficult and 'luck' based that they reused it on TAR 15 in a NEL.
Season 6's Leg 6 gave us not only a long double leg, but to finish that all up, they were required to eat a bowl of 'very' spicy Hungarian soup. Teams vomited and Freddy ate his own puke to beat Gus & Hera.
Season 6's Leg 11 was the penultimate, and the hardest challenge of the entire race thus far made participants bleed, and caused the elimination of a team. This was all due to the allotted time they had.
Season 7's 3rd leg presented a disgusting task that caused Romber and others to quit and take a penalty at the risk of getting eliminated and hoping others would save them by not participating.
Finally...
Season 7's 8th Leg required the eventual winners to shave their heads, affecting only Joyce. It was hard to watch, and uhhh.... they didn't win a prize that leg.