Nekofight would get my vote here. It's very simple, and yet, I find myself drawn back to it. The way the characters shuffle around is strangely hypnotic, and in spite of the fact that there is almost no gameplay, it's addictive to watch those little guys tumble around and fly off the platform.
I would say this qualifies as a completely bizarre game because, well... There's no explanation. And yet, it seems as if these two blocky beings belong here. It's their nature to tumble and tussle, to go flying to oblivion. The game's oddities aren't crammed in your face, they're just there. And you have to make of it what you will.
Well, maybe there is an explanation and it's just in Japanese like everything else in the game. If there is, I don't even want to know!