Gotcha Force.
In case you haven't heard of it, I'll make a mediocre attempt to describe it concisely.
You know how Pokémon Rumble (from what I've read) gave Pokémon an actual battle system?
You know how it (from what I've seen) was top-down?
In terms of the basic concept, Gotcha Force is kind of like Pokémon Rumble, only it's a third-person 3D battle system sort of thing and it has little robots called Gotcha Borgs instead of little animals called Pokémon.
You'll get a better idea of what it is if you look it up almost anywhere else.
Anyway, the game has infinite replay value for persevering perfectionists (like me) because the (up to 5) different colors of the Borgs are somehow easy enough to get in general that (at least to me) it feels like they're actually worth trying to get all of (unlike Pokémon's shinies), but rare enough that actually getting all of them would take several thousand years.
And the battle system is relatively simple in that your (up to 5) attacks (not including different melee attacks) can be done with 2 buttons, so fast-paced that GameSpot's reviewers call it "mindless button-mashing" even though it's highly strategic when you know what you're doing, highly varied with its 198 different normally obtainable Borgs, all with different strategies in their use (though some of them do have similar attacks to others), and more addictive than... something really addictive (in my experience).
I've beaten the game's story mode at least 20 times.
Other games worth mentioning include Tetris Attack (and similar games), Super Mario World (and the products of its hacking community), Super Bomberman (and, to a lesser extent, its sequels and other games in the series), and Super Smash Bros.