Phantasy Star Online Episode I and II (for GameCube). The hackers/modders ran rampant in that game, because all data was client-side-saved, since you could play the game offline and local multiplayer, and the same characters could be used back and forth. but that's what made it awesome. Everyone had the best weapons, armor, and upgrades, but the difficulty curve at higher levels was such that you needed all the strength you could to survive the monster gauntlets. Everyone had crazy modded weapons, and lots and lots of everything that was essential
Item duplication, hacking, stat hacking, all stemmed from the same glitch that was built in the game, too. So most of the modding didn't even require an Action Replay. Toss that in the mix, and you got a crazy experience.
Monster Hunter: pissing me off since 2010.
