Overrated:
Gears of War series. The gameplay was tight, but...something about it bored me.
Underrated:
Pokemon--Best RPG series ever. Not to mention, the Utopia of a ten-year-old is an ammusing and frightening setting.
Halo 3: ODST--Excellent atmosphere...it felt like the video game version of the Firefly episoe "Out of Gas" (I wonder if they sought out the Firefly cast for that reason?), and it demonstrates the strength of the medium.
The series in general seems to get a lot of hate here, despite having great gameplay--which gets polished with each installment, vehicle sections that are actually GOOD, and Forge Mode
Overrated
-Metal Gear Solid 2: people just don't get that Kojima made it more as a post modern experiment than a proper game, a philosophical joke at the player's expense.
Honestly, I'm putting it under "underrated" for the same reason. I didn't like it until I noticed the joke.
The Legend of Zelda: The Wind Waker/Majora's Mask--
Often, Twilight Princess or Ocarina of Time are hailed as the best 3D LoZ games, and these are fantastic games; however, I love the sailing in WW: it really amplifies the feeling of adventure sometimes lost while riding Horseback through a field, and I love how Majora's Mask turned the entire game into one big puzzle--In retrospect, I wish I hadn't used a strategy guide, but MM was brilliant.
Final Fantasy 9--Although gameplay elements doubling as huge plot elements is not rare in a Final Fantasy title (especially on the playstation), I really like how the villain's big plan is using the source of random encounters to start a war in order to obtain Summons, followed by learning to use a limit break. Doubled with the retro-style return to classic final fantasy, this is easily my favorite in the series.
Knights of the Old Republic 2--I really liked the first one; however, I am in the minority when I say Obsidian's sequel is better. Yes, it had some buggy moments, and a lot of content was cut--something that still has me pissed off at LucasArts. Basically, I thought the characters were better, the gameplay changes that were good, and the story--while lacking a huge twist--was still excellent.