This isn't the end all definition, but sometimes while playing old school games like Axelay, or Geometry Wars, I seem to enter a trance like state. My fingers just know what to do, even when my mind can't think that fast. Experiences like that are rare. I think games like that are hardcore.
RPG's, RTS's, well made platformers, and Metroid (except the ,still awesome, Fusion) games also fit the bill.