DtDust said:
Perfect Dark 64 and Goldeneye 64 have the best cheat system,
To unlock them you have to finish levels on a high difficulty in an insanely short amount of time.
Then while they are on you cant earn more
Gungrave: Overdose had a pretty good one, and you had to have ridiculous endgame stats for the best cheats, like the invulnerability cheat you needed to complete every level without losing any health (which is difficult to do even on easy since in later levels you'll be fighting wave after wave of 30 dudes and never have a chance to recharge your shield)
or you could just beat the entire game on Kick-ass difficulty (the hardest difficulty) and you'd unlock all of them, but I couldn't even beat it on Hard to unlock Kick-ass, one spray of tommy gun fire from a first level gangster was enough to lower your shield down to critical level, so I don't even want to think about what would happen on something harder then that (I assume it would involve RAEG and needing to buy a couple new controllers)