The game that started the series that largely defined the fantasy JRPG genre. With an emphasis on fantasy, since all the games of the series have taken place in a relatively standard medieval fantasy world.
I was introduced to the series only recently when I learned that most of the older Dragon Quest games had remastered versions for mobile devices. And I've played through most of them - including obviously the very first Dragon Quest.
The princess has been kidnapped by the evil Dragonlord and the only one who can save the land is a descendant of a legendary hero of the ages past. Perhaps today the story is as cliché as can be, but at the time, this effectively brought the classic fantasy story into a video game. The gameplay itself is simplistic but enjoyable in its own way.
Some practical elements: Starting from this until Dragon Quest V, the games were originally released on NES. However, the remakes have used later SNES versions. Were the games be released via GOG, I would personally recommend using the later versions, for not only do they have better graphics, some of them also have additional content the original versions of the games didn't have.
There is no doubt about it - a game as historically significant as Dragon Quest definitely deserves to be preserved. And the way I see it, other titles of the series as well.
At the time I'm writing this, a HD remaster containing both DQ and DQ 2 is scheduled for a multiplatform release next October. It would appear the series is finally gaining notable recognition outside of Japan.
Were it my decision to make, GOG would preserve both versions.