Dragon Quest is a series that's a massive hit in Japan, but fairly obscure almost anywhere else. Which might be why I learned about the series only relatively recently, and thanks to the Internet.
I played through the android mobile version of Dragon Quest III some time ago. It has just about all the mechanics and features one would expect from a fantasy RPG: A large world to explore, treasures to collect, leveling up characters, and character classes.
Especially notable is the fact that with the exception of the Hero, the characters can change their classes after progressing far enough in the game. In fact, changing the classes is encouraged, since the characters will retain half of their stats and all the spells they had so far learned, allowing for combinations that normally wouldn't be possible.
I can certainly see why this is the game that set the blueprint all the future titles would follow. And the reason it is the one that made the franchise famous in Japan.
Therefore, it could well be argued Dragon Quest III is also the game with the most historical value in the entire series. Certainly something GOG should have.
Although, I heavily recommend the version on GOG should be based on the SNES version rather than NES. Better looking character sprites, plus the remakes have additional features, such as the Thief character class and mini medals that can be collected and exchanged for special items.