The Japanese version Tenchu for PS1 (with cheats-on).
Besides the grappling hook mechanic (which allowed you to go virtually anywhere), I had so much fun messing with the stupid enemies AI. You could:
shank a dude
drop a poisonous rice ball next to the body
when his buddy walks by, he'll go into full alert for JUST A SECOND, until he realizes that there is a tasty treat in the dirt next to the body and eats it.
Sometimes, I'd do an old cartoon trick by making a trail of poisonous rice ball/sake. Then I'd watch from the rooftops as a guard would eat/drink one, fall down for a few seconds, get up, see the next one, and repeat the pattern. I also had fun dropping bombs on guards, blowing them up, watch them go on alert for a second or two, and then go back to their posts with their heads on fire.
Stupid North American release made the enemies all smarter. -_-
Ah Faxanadu! One of my favorite games to play at a friends house when I was a kid. Mostly because we both didn't have a CLUE on what was going on, and felt that maybe if we put our heads together we could win it.