I recently became addicted to Demon's Souls on PS3. It got Gamespots GOTY award. It's single player with funky multiplayer aspects. Fairly hardcore game. Highly addictive. It has a story, but it's fairly thin. It feels like Diablo, but not in the borderlands kind of way, in more of a "This is a dark disgusting world with evil crap that you need to kill and you can die any second and the loot is ridiculously hard to get/ figure out how to use" kind of way. And it has all sorts of interesting bosses/ locations. One of the games goals is "kill all of the 20ish bosses." (It makes it feel like what Shadow of the Colossus might feel like).
Borderlands was more of a "Team, Kill, loot, rinse repeat" game. It didn't have the fear factor, or the fear of random PVP during gameplay, which Demons souls has.