Nobody's ever heard of it; I've been singing its praises since 2006, and I've never met another person in real life who's heard of it. It's an amazing game set in a slightly-steampunk world where cars have only recently been invented, but giant steam-powered mechs were invented around the same time as well. The story's interesting, but the real fun comes from how much freedom the game gives in how you want to play it:

You can customize your character's clothes, you can be a good guy, you can be a jerk who charges his friends for every little favor, you can just straight-up be a villain, you can hustle pool, you can play in a band with a bunch of different instruments, each with their own mini game associated with playing them, you can extort or save an orphanage, you can buy and decorate an apartment, then play a dating sim with some of the characters, and that's all before you factor in the giant mech, which you can customize with a bunch of different pieces and use to fight in a colosseum, explore ruins for treasure, excavate fossils to save a museum, fight giant bosses, transport goods and passengers, and even turn it into an airplane to fly around in.

And that's all in a PS2 game! Sure, all of the features are limited by both the hardware and the inclusion of so many other features, but they're all fun, and the graphics look great. I rarely play any game more than once, and I've played this game well over a dozen times. It's helped by the different endings depending on how you play your character, but even the parts that are the same between playthroughs are still fun every time. It's my favorite game of all time by a huge margin.