Only one for me. Wasteland on the C64.
It was unlike anything else I played, sucked most of my free time in my mid-teenage years and created a backdrop that is used in many, many games up to the present day. When I realised that there was a Kickstarter campaign for Wasteland 2 - I pumped £100 into its development without really thinking about it. The follow-up won't be perfect, I know that. But I'll be looking forward to seeing how the background story develops and to see if a game in 2014 has the same pull as one from 1988.
The only other game that comes remotely close to perfection for me, was Dungeon Master on the Amiga. It needed the additional 512k memory expansion to run (the lucky buggers on the Atari ST got away without the memory upgrade) - but it set a very, very high and new standard for dungeon-crawler/RPG games. Many followed suit - the Eye of the Beholder series, the Wizardry series, Ultima Underworld series, etc - but none quite matched the original's WOW factor on release.
D.