Beyond Good and Evil.

Assassins Creed Origins (at least as good as horizon, if not better imo)
Tomb Raider Reboot series (better than uncharted)
Need for Speed: Most Wanted 2012 (better than 2005, imo)
Fallout 4 (better than fallout 3)
Sunset Overdrive
Pokemon Ultra Sun/Moon (best Pokemon games of all time)
FarCry 4 (better than 3)
Recore (definitive edition fixed up a lot of thing, now it's a solid 7.5 game)
Gran turismo 4 might be critically acclaimed, but I still say it's underrated due to people preferring 3 over 4 for some reason, while 4 improves upon 3 on almost every aspect.

Driver: San Francisco (one of the hidden gems of gen 7, especially with the car changing mechanic where you could get into and control any car on the street, and shifting in the driver's body)

Watchdogs_2 (basically what the first game should've been. The amount of things you can do with your expanded hacking options are infinite)
Captain Toad: treasure tracker (better than super Mario 3D world itself, imo)

I have way more that I won't list for now.

Geist on gamecube. The puzzles around the possession mechanic were pretty good and I loved the multiplayer. I really wish a sequel or another FPS used possession as a main mechanic (or at least the multiplayer).

i just grabbed the game on psn. good one i heard this many times

Soo...I googled a video for nostalgia...and lo and behold...part two is in the works. Def check out the teaser video on YouTube. Awesome...

They're a few. Don't think there is one above the other since a lot of games I think have been screwed over.

Puppeteer
Alpha protocol
Shadows of the Damned
Binary Domain

The Armored Core series.

Yooka-Laylee, Driveclub, Crash Bandicoot XS (The huge adventure), Modnation Racers, Rayman Raving Rabbids (original only), Spy vs Spy (PS2/Xbox), Super Mario Sunshine, Super Mario Strikers, Legend of Spyro: a new beginning (PS2), Blur, Need for Speed Rivals, Lego Racers (PS1/N64), Bugs Bunny Lost in Time, Crash Bash.

Not sure if that counts, but I would say some visual novels like Steins;Gate, or the Zero Escape series. I mean that it's usually highly rated, but very niche, so underrated or even completely ignored by majority of gamers.

Other than that, the most overrated thing I can think about is the multiplayer of The Last of Us, one of the best I've played, and most people don't even know it exists.

If we talk about older series, I would say Advance Wars. It completely disappeared and too many people forgot it.

Tales of Symphonia. It was rated well, but it should be like 10/10