Essential if you own a Commodore 64
The Swiss Family Robinson for Commodore 64
I have such fond memories of this game. The cheesy early Commodore 64 music, the bright and colorful graphics (which may look crude now, but were great looking back then), and the depth for a game so old is amazing.
Basically this game is a mixture of a text-adventure and a puzzle game. You explore a remote tropical island surviving various traps and wild animals and such. All the while you are searching for items such as firewood, food, and boat making supplies as well as clues to how to escape in order to survive the harsh conditions. This game was complicated enough to have changing seasons which affected your character, in game events, and an actual storyline as well as other characters you could interact with. None of that sounds like a dang thing nowadays, but back then it was groundbreaking. Most people were still playing Pac-Man and Defender.
I spent weeks fully immersed in this game, something that was nearly unheard of before the NES. It was a process of trial and error, over and over again. No walkthroughs. No strategy guides. No online guides. And this was long before there was any overused or cliched game gimmicks or secrets. So you were literally figuring everything out from scratch. It was a hard, yet satisfying experience that I didn't really get again until I got my NES.