Either SplinterCell Chaos Theory or Resident Evil 4.
Both to the point of insanehandicap runs. SplinterCell i did a "no run, 100%, no gadgets, no equippement, no wistle, no goggles run" on hard.
And RE4 did everything from a handgun only, knife only, grenade only and imo the hardest of them all 'punisher only + no knife' run(s). And ofcourse speedruns. Damn I love those games!