Suikoden.
Ys.

Mario and gta

Freedom Force. Everyone should play Freedom Force.

Also, a while ago there was this game called Demon's Souls that I felt was greatly underappreciated, but I think the dev team is getting the appropriate amount of appreciation now.

The Legend of Heroes. The Trails games in it are some of the best JRPGs I've ever played, and while they do seem to sell fairly well for what is obviously quite a niche franchise, they deserve more.

Syphon Filter.

Nioh and Nioh 2, the most underrated series in the world. It is worse now with Chaos being a mediocre game.

Persona 5 gets a lot of praise but Persona 5 Strikers doesn't get much praise, despite being an amazing game.

Dynasty Warriors series, mindless fun in games is rarely praised.

RTS games in general are unpopular despite the depth of gameplay.

I think Skies of Arcadia should be more popular. Just popular enough to earn a re-release on modern systems.

My vote would go to Chibi robo. Amazing series imo.

It's not quite the same thing, but every so often, I forget that although it has a significant online presence, Ace Attorney is a fairly niche series whose average release only pushes between 250k and 300k sales. And the entire Ace Attorney franchise combined has yet to sell even 9 million copies.

Then I go "Oh, that's because a series of mystery-themed visual novels often only released in Japan is never going to be THAT big a seller."

Then I remember that the JB Harold series sold 20 million copies by 2011, despite being "a series of mystery-themed visual novels ALMOST ALWAYS only released in Japan." And then I wonder what the hell is even going on anymore.

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Besides that, focusing on games with low sales as my main metric of determining what is "underrated?" The non-Persona Shin Megami Tensei games, Bayonetta, and strangely, the original Doom duology (surprisingly, VGChartz indicates that Doom and Doom II, two of the most important shooters ever released, sold a combined 3.5 million or so units thru 1999, about as much as the PS1 version of Rugrats in Paris).

Metroid
Mana y Saga rpgs series