Katana ZERO for gameplay and Oneshot for story, so they are both tied for me.
I have sunken the most hours into Siege and Minecraft, but the heart wrenching story of them and the beautiful art, game design, and thought and care put into them make them my top picks, along with replay value.
With Oneshot, there's is a whole other game in oneshot after you finish it, and the puzzles while maybe not being too difficult are unique and like nothing I've seen before. Shit made me cry.
And Katana ZERO has a great gimmick to draw you in, but the fast action and precise and tight fighting mechanics hold you in to stay. legit some of the most ass-clenching game play I've ever gotten from a single-player experience.
And to boot, they are both dirt cheap, making them worth more than you'll pay for them.