I'm so glad I grew up past some of the more infamous ones before saving was popularized.
Most Tedious: runescape skilling and WoW's sea turtle mount
most flipdesk frustrating: half life 1 black ops ninja fights and Xen jumping puzzles
most challenging: runescape's Nomad boss fight on release. Later updates indirectly made it easier, but on release it was the toughest thing in the game. One boss fight, one week straight of attempts, trying dozens of strategies. He went down eventually, but I was getting ready to hang up my cape.
It's never a game, it's always that one level.