I'm going to run with saying Eternal Darkness: Sanity's Requiem on Gamecube. It's really one of those games so unique you can not define yourself as a "hardcore gamer" (as opposed to a casual gamer) without giving it a swing. It relies on a concept that never had been used before and never will be used again at this rate (the sanity meter), it has a solid story, a fantastic collection of characters with varying weapons and stories without feeling over the top and is, in general, a very good game.
I'd also say Rune: Viking Warlord on PS2 is another great game worthy of notice. While it's not a game that you must punish yourself for not buying, it's a great game that feels manly when you play without going over the top. Plus, the progression of all the different weapons, armour and shields is great. There is plus a nice variation of environments without going over the top. From running around in a cave, trying to cut little goblins out, to urban Dwarfish environments, to Hel to the very cold and, well, snowy environment. If you ever want to see how a Viking game is done right, this is your Valhalla.
Another great unknown/under-rated game worthy of notice is Mount And Blade: Warband on PC. While there is a severe lack of a story to it (which may feel to some like eating a 99 ice-cream without a flake in it), it's made up for how nice and realistic it is. Sure there is still an annoying health bar metre and it's got nothing on Bushido Blade (which I may as well mention), it's still is realistic. Not always in the large bigger picture but just the many little small things. Things like men acting sexist towards female soldiers and the ability to alter the difficulty to take the same damage as the enemy soldiers. I wrote a review about it if you want more. Plus, there is a demo of the original Mount And Blade. Just play that if you're not sure and if you like it, just get the Warband version with more things and a multi-player which, predictably, is somewhat unbalanced but is also fun. It's also one of the few games that I've had to stop playing just so I don't burn the entire day playing it or because it's 5am and I really should go to bed.
Another game worth mentioning is Bushido Blade on PS1. Whatever you really thought was realism in sword fighting, Bushido Blade beats that and then rubs it's face into the mud like a 20 year old bully in a primary school playground (no, it's not a paedophile, it's a guy who physically grows up faster due to a genetic disease, don't worry). It's got no health bars, what weapon you chose will affect how slow you swing the thing or how you generally handle it. It's got a honour system for crying out loud to punish those idiots who feel the need to kill their enemies by stabbing them on the floor before they get up. Pretty cheap to pick up as well.
I better stop there before I turn this into a mass essay about all the different games and how they only ended up a worse game in it's time due to the lack of publicity.