Underrated:
Ooh, and I can't belive I forgot this: Shadow Hearts. There are three games in the series, and I think 2's the best (and the easiest to find). It's such a fantastic series; a gothic, historical horror-RPG with an involving plot, absolutely amazing music, a unique battle system (<3 the Judgement Ring), memorable characters... and yet nobody's ever heard of it.
*spaz* Someone else has heard of Shadow Hearts? So there is a god out there! I actually had a hard time finding 1 & 2. 3 was always right there for me o.O
iknorite? Phantom Brave, Disgaea, Altier Iris and Ar Tonelico are all pretty good games, but underrated as hell.
Well, I'm not exactly sure, but I think Xenosaga is pretty underrated. I'm not sure if it's well known or not, though.
NIS games don't get that much recognition, unfortunately. But Atlus' publishing of the original Disgaea for the PS2 did manage to get NIS enough money to establish NIS America, which in turn, let us witness NIS' other great titles, like Dragoneer's Aria, and such. My favorite titles by them so far have to be Phantom Brave, and both Mana-Khemias.
Xenosaga isn't really underrated, and it's well known, it just gets ALOT of hate from people who grew up with Xenogears. It's a decent series, but I'm sure if Episode 2 hadn't flopped, and Monolith's writer hadn't quit, we would've had the original pretentious, 6-part epic it was originally supposed to be =/ And Episode 2 wouldn't have been so mediocre.
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Moving on! My own under/over rated stuff?
UNDERRATED:
Definitely the Tales series, Shin Megami Tensei, and nearly any 2D fighting game outside of Street Fighter. Specifically Darkstalkers, Killer Instinct, and The King of Fighters seems to have lost a large portion of its fanbase, after SNK went bankrupt back in 2000.
Tales: Unfortunately, people not knowing about it, or giving it recognition it deserves isn't enough. The series basically doomed itself from western audiences, by not localizing any of the games with varying casts of characters, non cookie-cutter stories, and varying battle systems. We were lucky to see Tales of Phantasia in America, granted it was horribly translated, and it was the worst version of the game, but it was still solid. But we've been denied release of any other games that would really make an impact on fanbase, like the remake of Destiny, or Rebirth even. Namco further showed us its stupidity after declining XSeed's offer to localize the PSP port of Tales of Rebirth...
MegaTen: Persona 3 seems to have blown the fanbase for this series through the roof, but it seems like no one knows about Shin Megami Tensei, or Digital Devil Saga. the Raidou Kuzunouha games are also solid entries in the series, and yet, don't seem to get enough recognition =/
Fighting games: Darkstalkers is just one of Capcom's forgotten series. Killer Instinct is in serious need of revival. Banjo-Kazooie on XBLA sorta gives me hope though. And SNK seems to be making a comeback now at the very least. KOFXII will definitely be a nice breath of fresh air for the series, especially since it's the first time they'll be using new sprites in the last 15 years.
OVERRATED:
Final Fantasy XII, and Resident Evil 4 & 5.
FFXII was just horrible. And it plays too much like an MMO. I don't like MMO's very much >(
Resident Evil 4 & 5: Come on, it's not even Resident Evil anymore. They're just another action game. Capcom would be better off making another God Hand (Which is an amazing game, btw)... Oh, wait... Madworld did that already >.> It's not just that, but RE5 just screams Metal Gear Solid 4 rip-off. The entire atmosphere and everything. And the controls are crap too. 4 was at least decent, and fun to play, especially on the Wii, and it had Leon, but I seriously hope Capcom doesn't remake Resident Evil 2 like this, IF they decide to do it.
((EDIT: I apologize for the novel, but I tend to rant about my games >.<))
EDIT AGAIN: Another underrated game: Klonoa. Sure the gameplay's simple, and the stages and bosses are really easy, but the stories are always so innovative, humorous and emotional.