I am not entirely sure how obscure it is, but Gotcha Force for the Nintendo Gamecube.
It's a fairly simple game. You simply assemble a team of any of the characters you have unlocked (known as borgs) and you fight enemies until either their force or your own is wiped out entirely. Each time the borg you control is defeated, you respawn on the next one in your force.
Each borg has simple attacks (only two buttons, possibly with charge functions or alternate attacks at close range), but the variety comes from the sheer amount of characters. While a lot of them function similarly to some of the others, there are over 200 possible characters, ranging from samurai to soldiers to jets to tanks to knights to dragons to bugs. And some of them are flat out hilarious. One is the Walking Bomb, whose sole purpose is to grab an enemy, pull them in, and then explode, killing himself and dealing probably lethal damage to everyone around. Then the Death Eye, which is a slow character with one jump, one point of health, an attack that fires a single, low-damage bullet and who dies in one hit from almost everything. Then you unlock the Death ICBM. It's the Death Eye, but riding a missile. Which you then plow into the ground to make an explosion that deals insane damage to everything over most of the battlefield.
The game had mediocre reviews and almost no marketing, but even if it's not technically a great game, it kept me and my friends entertained for quite some time. And at least in my searching, it seems to be impossible to find at anything less than collector prices outside of Japan.