The game I think is the most underrated is Beyond Good & Evil. Released in 2003, it was a commercial disappointment at the time but has since gained a cult following. The game blends action, adventure, and puzzle elements seamlessly, with a rich story and engaging characters. The protagonist, Jade, is a strong, relatable character, and the game's world is beautifully crafted with a unique art style and intriguing lore. Despite its initial lack of commercial success, it remains highly regarded by those who have played it and is often cited as a hidden gem in the gaming community.
The Custom Robo series in general. The first two games came out only in Japan. The Gamecube game had fewer sales than Japan I think, and the DS game I really like because it has online connectivity. It's an action RPG in a way, but it's also like a fighting game with projectiles. There are so many missions, and achievements. The selling point is having custom parts for your robot to fight others with. The futuristic setting where the robots use quantum computers. Even criminals use Custom Robos with illegal, overpowered parts. There's a lot of thought into the games. They never made another after the DS. Synaptic Drive for the Switch wasn't as good. But Custom Robo would need a lot of effort into it to make it good.
Sonic Unleashed and Sonic Generations. Everyone hated the Werehog transitions but I thought it was something different and the combat was actually easy to pick up. Generations everyone hated because it was 2D/3D, I loved it.