Rules -  The game cannot have critical success, it could sell a million, but if the horndogs at ______ (insert big name game site here) said LOL it's bad then you are free to list it, but I've seen people try time and time again to list games like SMG, Uncharted 2, and Mass Effect, and those just simply aren't underrated titles.

Try to make a short description of the game so people who never played said game might find a reason to buy it.

I've got a ton of games I would list but it's late, trying to kill some time so I can fall asleep so I'll name off a few and flesh out my list later.

Overlord Dark Legend - Wii - Hilarious game, part action, part RTS, and to me had much better controls and more interesting than Pikmin NPC that came out around that time.

Squeeballs - Wii - It's another Party game, it's not the best, nor does it live up to Wii Sports, but man its got some character, and I like it for its humorous moments. 

Project Slypheed - 360 - Space shooter, it controls a lot like Star Fox in all range mode, but it plays almost like a SHUMP, and its just a ton of fun, and has a lot of ship customization that I enjoy

Earth Defense Force - 360 - So much pure awesome packed into one TPS, its co-op, its hard, and its like playing through an Action B-movie story, giant ants and spiders, the works!

Naruto UNS - PS3 - Simply the best Brawler outside of Super Smash Brothers, and easily the best anime based fighting game, its easy to get into but once you figure things out it becomes hard to master, especially if you play skilled people.

-will add more DS and PSP stuff later-

1. Wii Sports.

2. Wii Fit.

3. Wii Sports resort.

4. Mario Kart

5. New Super Mario Brothers.

Edit: JK of course!

Obviously, people do not rate them as highly as they ought to even if they do not like them.

Edit: What do you mean by lacking critical success? Under 90? Under 80?

-saints row series - underrated in comparison to GTA (am i breaking the rules)
-Viva Pinata
-Condemned
-Samurai Warriors 2

Enemy Territory: Quake Wars is a fantastic game even on consoles where it got something like 65% Meta. Awesome team based, advanced shooter, greatly balanced etc.

Dark Messiah of Might & Magic (72% Meta on PC, console version lower) - Superb melee action game in first person. A great adventure. Simply a great, almost fantastic game.

Wolfenstein - rufly 72% Meta all platforms. Is as good as any top notch FPS. Atmospheric. And it's Wolfenstein!

Star Wars The Force Unleashed (~72% Meta)- not a superb game but clearly underrated since it was a solid hacknslash with a nice story and environments.

CRACKDOWN
Lost Odyssey

I really liked mirror's edge and its first person platforming, it was rated ok but i think more games should try that first person parkour thing

Mirror's edge
Sin Episodes

Dante's Inferno-I liked the games atmosphere. I think the game is highly under-rated and got an unfair rap.

1. Muramasa: The Demon Blade - Made by the small studio Vanillaware and from the seldom used genre of Side Scrolling Action/Adventure title often populated by Metroid and Castlevania titles, this game for the Wii was well received (for a Wii game) but has to date only sold about 200,000 units.  Clearly one of the best titles on the Wii, it should do much better and is one of the best games that has come out this gen in terms of gameplay and pure art design.

2. Kirby's Canvas Curse - Near the launch of the original DS, we were given a new Kirby game.  But not one like a normal Kirby platformer.  Instead, this was an innovative title, where you controlled Kirby in a ball form and helped guide him through a world full of pitfalls, enemies, electric fences and lava walls.  The controls were simple but the gameplay was complex and addicting.  And most of all, it was fun.  It sold well for both a new DS title and a Kirby title, but few people look back to this title in the DS library since it came out so early.  When its still one of the most innovative and fun DS games to be made.

3. Dragon Quest V - Now this title is purely based on western opinion.  Clearly this title isn't underappreciated in Japan, but here in the west, its hardly even known.  And its a shame, since its easily one of the best RPGs on the DS without question (and I would say one of the best RPGs out currently).  But based on its dismal 100,000 unit sale in the west, and that mostly due to the total lack of marketing from SquareEnix, the vast majority of people outside of Japan don't even know it exist.  Its really too bad, as this RPG has something for nearly every RPG fan and can trace its roots back to the beginning of many key RPG franchises.

easy....Final Fantasy XIII