Either of the Knights of the Old Republic games, though a big reason for me is the storyline. It's one of my favorite campaigns with characters I adore, especially Kreia in The Sith Lords. The combat isn't hugely innovative, but considering BioWare put out Dragon Age: Origins just a few years ago with a similar turn-based combat and the fact it's still a prominent combat style in certain genres, it's not anything with which people would be unfamiliar. Then I just plainly enjoy BioWare's role-playing elements, with a morality system stemming from choices that affect quests, party members and abilities to an extent, and enough customization to where I feel like each character I roll is significantly different, so long as I'm so inclined as to make it satisfyingly different from prior characters.