I played a lot of games when I was a kid, but there were two that stick out in my memory that when I got home with the games and loaded them up that was it. I stayed up all night playing them till I got them beat.
Elite on the Commodore 64. Elite is one of those rare games that there were many different versions of the game with many different features. It's a game that has been done for nearly every computer platform in existence & each one of those versions has varying degrees of features. The Commodore 64 version was one of the most feature-packed or the most feature-packed as was revealed in a review I read with the original programmer of the game.
It was one of the largest, most complex games ever. Definitely for its time period and in some ways even today it is unmatched. You started out with a small modest spaceship at a random space station in a galaxy full of hundreds upon hundreds of planets. Each of these planets had its own background and economy. You could decide to haul goods from port to port for trade to make your fortune or take on missions such as escorting ships and tracking down criminals. That's if you wanted to be a good guy. If you wanted to be evil there was plenty of contraband to smuggle from drugs to slaves. Or if you preferred the gimme approach you could just load up your ship for bear and take to piracy on the space lanes blowing up ships and taking their cargo by force.
If you by chance got bored with all those planets or wore out your welcome there you could later buy a hyperdrive and travel to seven other galaxies each with just as many planets as the first one.
You started out with a ranking of Harmless and as your exploits progressed you could become Mostly Harmless, Dangerous, and a bunch of other rankings all the way up to the ultimate ranking of Elite. Once you became (in)famous enough the military comes looking for you and you get the opportunity to do top-secret military missions and get access to military hardware for your ship.
There was even an alien race for you to fight. They'd attack at random times or sometimes your hyperdrive would misfire and you'd land in a nest of them. Destroy them and you'd get alien artifacts to sell and alien gear for your ship.
And finally there was the easter egg. The mother of all enemies. There was a ship from an unknown alien race that was traveling the galaxies that you may or may not ever run into. The cool thing about this ship was it was cloaked. Invisible. Track down that ship, manage to destroy it, and the cloaking device was yours!
I managed to achieve the ranking of Elite which was not easy. You had to literally kill something like 100,000 ships or so to achieve it, but I never did grab that cloaking device.
The second game was Dungeon Master on the Amiga. People who played in on the Atari ST had a similar gaming experience, but the PC version was crap due to limitation in graphics and sound on the PC platform at the time.
There's not as much to say about this game as Elite. Not that it wasn't as good, but it was not as complicated a concept. It was an epic dungeon crawler with state-of-the-art graphics and sound for the time. Take your party of four down 25 (I think that was the number) levels and defeat the Evil Bad Guy at the bottom. Inventive monsters, traps and puzzles made it an instant classic at the time. I replayed it a few times with different party combinations and tactics to make sure I didn't miss any secret areas or items.
Like I said, I played many games, but those are the two that stick in my mind as being instant play-till-dawn obsessions.