Ninja Gaiden II's Mission Mode - A lot of the missions are crap, but whoever designed the survival missions is an absolute genius. I'd probably have his babies.
All of Cave's DLC, since they're entire games for $15 (or free as a first print bonus). Futari Black Label is the best because it's the best game ever
Dark Souls PTDE - Best boss fights in the game.
Sayuri in Hard Corps Uprising - Turns the game into Strider.
Side Vower in Strania - It's basically an entirely new game for $5.
SF4 Arcade Edition - 4 characters and two balance updates for $15. Not bad.