Ok, looks like we need a new word for great games that sold poorly, as underrated doesn't quite fit, and actively seems to offend some people.
How about underhyped? We've talked about how overhyped games have been in the past, Surely that's the main problem with the Katamaris, God Hand, Psychonauts, Colossus, etc.
As for the hate on God Hand, it took an old style of game, the 'Final Fight' side scrolling beat em up, and applied a shiny new update and some interesting fight styles to it, along with humour and a fun soundtrack. The reason a lot of people don't like it is that it's not pandering to the lowest common denominator by being easily winnable in a few hours, you a need a bit of skill and good reactions to beat anyone. No it's not perfect, but its a good example of how to update a classic, underused sub-genre of game.
Of course I still think SF2 Turbo is the pinnacle of one on one fighting games, to me it had the perfect level of complexity to bring strategical play into it, while still being instinctual because the moves list didn't come in a set of leatherbound volumes.