SaGa Frontier. Nobody but me and a buddy of mine seemed to dig that one, and every game since in the series has been just above (or just below for Unlimited SaGa) garbage, so no one gives this game any love. It's far from perfect, but it has tons of great characters, interesting locations, solid music... and best of all... each of the 6 (or seven?) characters' stories are almost impossible to beat because the game dicks you so hard in boss fights!!! ...well, maybe that part isn't so great, but I still love the game.
And I know someone said it already, but man I loves me some Armored Core (preferably Last Raven for the PS2). The fighting system might kind of suck, but I love tweaking the attributes of my parts, seeing negligible results, but still feeling like a badass.