The Monkey Island scheme was an obvious highlight, however I quite like the one from Railroad Tycoon where you had to visually recognize the trains using the manual. As a bit of an anorak I memorized them all (it was quite easy), effectively defeating the mechanism. There were so many cracked versions going around I don't know why most companies bothered.
Btw cracked version of Railroad Tycoon was better. Not because copy protection was removed but because they packed game to one disk. The original cames on two disks.
When it comes to those "play guessing game with your manual" kind of copy protections then this one used in Railroad Tycoon was the best approach. You couldnt know those trains without playing the game. And you couldnt play this game much if you didnt succeed with copy protection. But fater few games you could indeed remember those trains easily and copy protection was not on your way anymore.
