I'll start with a few that aren't underrated so much as unknown.
Lack of Love - Sega Dreamcast -- Possibly the most charming game I've ever played, sitting alongside NiGHTS into Dreams..., Chu Chu Rocket!, Jet Set Radio, and Rez in terms of games that instantly and effortlessly evoke my adoration. It's a very quaintly made game by a relatively small team of ex-SquareSoft employees that explores the nature of life, evolution, and responsibility through a progression of cute and strange creatures and levels. The fact that it's so overlooked is criminal, considering how subtly it handles a theme that would become overbearing and trite in the hands of most game designers, ultimately achieving a sweet-spot right at the intersection of Pikmin, Seaman, and EVO: The Search for Eden (on Super Nintendo -- another overlooked classic to search out!).
Download 2 - PC Engine CD -- This import PC Engine shmup is a favorite of mine for its incredible atmosphere as much as its solid gameplay. Imagine a cross between Rez and Snatcher and you've totally got it. A little bit tricky to locate online on account of its extremely common name (in its defense, I'm sure it sounded very unique and high-tech back in 1991). You're a computer-hacker with a fighter-jet that can warp into cyber-space, on a mission through the international super-computer's glitched-out memory banks of visual files which span throughout the course of time to retrieve Hitler's brain from a group of terrorists who wish to use it in their scheme to take over the world. Never before have I found a game that played to my gamer's heart of hearts like this one did.
Undercover Cops - Arcade/Super Famicom -- Go for the Japanese arcade version, which has more features and a better soundtrack. This is one incredible beat-'em-up that plays amazingly with the post-apocalyptic environment and themes that it sets up. You fight against scraggly junkies and literally insane bosses who use the decaying urban environment in increasingly imaginative ways to kill you. I won't spoil the ridiculousness of the low-level health power-ups for you, but they play both humorously and unsettlingly well into the post-apocalypic theme, and Rosa's "yummm" sound that goes along with it is hilarious. Great, solid gameplay and moves (Rosa even uses a move reminiscent of Honey's infamous butt attack from Fighting Vipers) combined with incredible art direction and tons of creativity to create an incredibly fun and entertaining beat-'em-up.
Rakugaki Showtime - Sony PlayStation -- I think it's a great testament to the taste of Sega fans that Treasure is one of the most renowned developers in the scene. Unfortunately, it also means that the vast majority of games they made for any first-party other than Sega were grossly overlooked (thankfully Sin & Punishment notwithstanding). Rakugaki Showtime is just such a game for the Sony PlayStation. Released by the company that with Yu Yu Hakusho Makyou Toitsusen and Guardian Heroes invented the free-for-all model of fighting games that would later be capitalized upon with Super Smash Bros. and Power Stone, Rakugaki Showtime employs that same manic, unpredictable action with an extremely cool 1st-grader art style, with deliberately erratic crayon-drawn animation over paper cut-outs fighting in a 3D space. The character designs are extremely varied and imaginative, and the gameplay is completely unique despite its general similarities to other party-fighters.
There are countless Sega Saturn and PC Engine games that could qualify for this list. In the case of the former, you're probably already aware of most of them, in the case of the latter, that's an adventure best taken on your own with the help of some videos and guides -- lots of fun explore. So many great titles, most completely unknown in the West.
-- Underrated. These are games that often garner low scores that I don't for the life of me understand. --
Opoona - Nintendo Wii -- Considered childish for its character design and imperfect translation, this is an incredible showpiece for charming video game world-building. It eschews the high-fantasy themes of most RPGs, along with ham-fisted good versus evil plotlines, in order to craft an unbelievably dense and intricate world of its own, complete with deeply contemplated works of totally foreign art and architecture for the player explore in between explorations through its lush and imaginative terrain. Imagine a cross between Panzer Dragoon Saga and Earthbound and you're not too far off.
Toy Commander - Sega Dreamcast -- This is an incredible title that was mostly adored upon release, but has somehow slid into the "hasn't-aged-well" territory for most gamers today -- gamers who are retro enthusiasts, at that. I have to say, I can't comprehend it. The graphics are still among the most intricate and inspired of any game of the era, crafting unbelievably cool little worlds and levels out of everyday household objects. There are neat secrets and Easter Eggs hidden around every corner, and it's simply an unbelievable testament to the creative energy being poured into the Dreamcast at that time. The various household objects turned into towering structures and unique play mechanics for each toy war machine come together to create 50-something levels with countless new ideas thrown at you in each new room. Outside of a few racing levels, the developers almost never repeat the same idea twice. Many see this as a weakness, and deem Toy Commander a jack of all trades, but master of none, but I staunchly disagree -- its theme is its versatility and incredible ability to create a world of possibilities out of the mundane. Also, being from French developer No Cliché, it stands as one of my favorite European-developed games of all time.
Panic! - Sega CD -- One of a mere handful of games to get the FMV idea completely and utterly right. What is traditionally seen as the downfall of the FMV genre -- press some button or another and whatever happens happens -- was somehow translated into a genuinely entertaining and hilarious experience. Admittedly, it sometimes sets the bar a bit to high with some amazingly ridiculous visual puns (SPOILER ALERT: the greatest of these that I've come across so far being a potion which covers your character's dog in hundreds of female breasts), and not all of the gags can keep up, but they do so often enough that you forget that you're supposed to be all sour because you can't control things -- the point of the game is that you're not in control, and that's where it pools its humor from. It's critical of mistakes that you have no way of avoiding in a very unique and humorous way.
Mister Mosquito - Sony PlayStation 2 -- This one was probably ranked fairly, scoring just a little bit below average, but it personally appealed to me greatly because it embraces the unmistakable Dreamcast feel of game design, from the graphics, to the off-the-wall ideas and humor, to the arcade-style gameplay translated into a presentation and setting that could never work in an arcade. Only problem was that it wasn't a Dreamcast game. To their credit, the gaming media actually did point this one out at the time for its "Oh my God, look how weird and Japanese" qualities (and deservedly so), but I can't help but think that this one would have turned into a real cult classic had it appeared on the Dreamcast.
Chu-Lip - Sony PlayStation 2 -- If you're like me, one of the greatest things about video games is (was, outside of very few titles today) the way that they created a comedy language all their own, based on thrusting the player into utterly nonsensical situations. Being a language tutor for Japanese speakers learning the English language, I've quite naturally found that they really are not so insane as the stereotype would lead one to believe -- however, they do have a very unique sense of humor that is largely rooted in a sort of visual situationism. Much of their humor derives from placing the audience in a situation that is utterly bizarre, and laughing at the idea that there is no rational response to such an insane situation. It ties beautifully into the psychological components of humor: stress, re-rationalization, and relief. Of course, sometimes it goes to far and those not in-the-know simply become psychologically disturbed by the material -- to me, these "mistakes of humor" are just as appealing. This has all been very roundabout, but Chu-Lip does this extraordinarily well, and rides that line between innocent humor and legitimate moments of not being able to interpret some information and being completely baffled. I've laughed my head off to this game many times over, and have been very pleased to see it get a little surge in popularity thanks to Game Grumps. Wasn't well-received in its day, though.
