1. Dragon's Lair*
2. The Legend of Zelda (1 & 2)
Past this point the answer has more to do with what I have available to me at the time. Games like Fallout 4, Diablo III, Portal etc... have all consumed a lot of my time over the years but when I finish playing them I'm fine. The two (technically 3) listed here have withstood the test of time as focal points of gaming for me. These are the games that always come to mind beyond any scope of nostalgia or current shiny release. Their my desert island games... Not that I would object to any of my other favorites showing up, it's just that these are the only ones with set in stone places on my list. I have bought, re-bought, emulated, drawn, mapped these games. I have even literally ate, slept and drank their spin off merchandise many times over the years.
*This game set a high bar in every category, other than direct control, it is little more than a cleverly disguised disc menu in that regard (But at the time, when random access video was virtually unheard of, it was seemingly magical.) It took 10 years just to get all the levels at home. 14 years to match the audio and come close on video. And 24 years to become nearly indistinguishable at home from the arcade. This was the game that made everything else pale in comparison on one level or another for most of my life and it came out the same year as Tapper, Spy Hunter and The original Mario Bros (not Super) hit the arcades. It was beyond futuristic and colored my opinion of what video games could be for many many years. I have collected this one on nearly every system I own and have bought systems just to experience their interpretation of it so I would be remis to not place it at the top of my list.I can not overstate the effect this game had on me as a kid.