Dark Messiah of Might and Magic
Was underrated, and today is pretty much forgotten. At the time it DID have performance-related issues (frame-rate issues, v-sync-related issues and stuttering to name those by heart, which I experienced myself, but was never a game breaker per say) but unfortunately many couldn't get past those to enjoy the actual game.
And the underrated part wasn't the story, which itself wasn't exactly complex nor full of mind-blowing and unexpected twists (although it wasn't bad either). It's the game-play that was fun, the melee combat system within first-person view wasn't common back then (it wasn't just a single melee option, it was things along the line of kicking the enemy on a wall of spikes, using the environment and enemy-traps against them). I also liked that the game was fully voiced in both English and French (and maybe other languages that I'm not aware of). If you switched your Steam's UI language to French (for example) then upon launching the game it would switch to its French version (with voices). That was a neat thing and I played both versions to the end (which also allowed me to choose different paths when I could).
I mean Dark Messiah in my book isn't a masterpiece, I'm not praising it like I would for something like Fallout 2, Baldur's Gate 2, Mass Effect or Super Metroid. But Dark Messiah was underrated, still is today by those whom can still remember that it even existed. And side note, it's the same guys that made Arx Fatalis and more recently Dishonored (they also contributed to the development of BioShock 2), namely Arkane Studios, got respect for them.