Halo 3 is better than... Crackdown
People don't enjoy Crackdown for its story, characters or setting. Nor do they particularly love the game for its combat or its vehicles. No, the number one reason anyone plays Crackdown is for its co-op. Two friends+ open, flexible world+ big guns, fast cars, rag doll pedestrians+ plenty of experimentation = one of the most amazingly addictive experiences available on Xbox 360.
Unfortunately for Crackdown, Halo 3 just multiplied that equation. No doubt you're aware of the beefed up co-op, supporting four players instead of two, but have you tried Forge? By enabling more than a dozen folks to simultaneously tweak a map, as well as kill each other while doing so, this overlooked editing mode creates a wild and random element of fun. In other words, everything that made Crackdown worth buying in the first place.