Best - Geo-Mod (Red Faction)
Almost no one still uses Geo-Mod as it existed in Red Faction, and with good reason: it's almost impossible to design your game where players can just blow up a wall or a floor instead of going on an extended mission to meet an NPC to get a key to unlock a door. But the brave folks behind Red Faction pioneered the incredibly awesome, if not particularly convenient for developers, gimmick. Waste time blowing everything around you up, find crazy new ways through levels and missions, defy physics by knocking down a load-bearing wall with no effects? Doesn't get much sweeter than that.