Generals (Zero Hour) is the first game for me in several things. It's the first Command & Conquer I played, first game I ever downloaded mods from and first game that I ever modded.
From all the games that I've played over the years, there are few that I can come back to and still enjoy as I did back in the day. It's graphics aren't great by modern standards, but they feel 3D and besides the lack of bloom and polygons they're not that bad. Gameplay tends to rely on small armies and if you get sick of the war on terror theme, then there's tons of mods to change the game to many other themes or even completely alter its gameplay altogether.
Generals is, and probably always remain to be an outlier to the series. It's not part of the 2 main series, nor are there construction yards. Instead Generals uses construction dozers, introduces generals powers, and laid the groundwork for the Command & Conquer games that followed it.
I would want Generals on GOG, not because it is the best Command & Conquer. But because it is an important part of the Command & Conquer franchise. And because I feel GOG could bring it the longevity it deserves, GOG can protect the game from EA's launchers that keep breaking mod support as GOG cares about the games and not the launcher.