When one gets right down to it, there's not a lot of room for opposition to this idea--that is, G Gundam is the most-original Gundam series of all. Not Gundam W, not Gundam 00, not even the cocaine-induced drama of Turn-A Gundam can compare to what G Gundam offers to fans of the traditional Gundam universe(s), so grab your mobile suits, Gundam fighters...READY, GO-OOOOOO!
From the very beginning, the works of G Gundam are fresh, new, and unhindered by the tired cliché of Gundam series; watch the first episode alone, with Domon Kasshu adopting the imagery of a lone wanderer and coming to a bar for some sort of refreshment, and you will see that this is not the series of its predecessors, and even sixteen years later, there's no successor series which has dared to do what it has dared to do.--Excessively flamboyant mobile suits, a world devoid of any combat models and non-Gundam-models in-total, character design worthy of a one-on-one fighting game with each being exciting entirely on its own, and a backstory which is amazingly interesting while simultaneously being relatively unnecessary to comprehend why the story playing-out on-screen should be entertaining. Beat that, Gundam series.
In this sense, G Gundam could be nominated for being the best Gundam production outside of the original, and there are few which could really take it on and expect to compare. Every other major Gundam-based series draws from the original's inspiration in the themes of war, conflict between peoples and families, the inherent superiority and "specialness" of Gundam-type units, and a main character whose youthfulness is ultimately his main obstacle. Ultimately, that makes Gundam series as a whole unoriginal even when compared to one-another and excluding the Universal Century timeline. But in-truth, G Gundam earns itself the rank of being the best Gundam production outside of the original because it indeed follows in the steps of its predecessor and creates something very, very, new.
The colonies of space are in-conflict with the people of Earth--yes, just as the original Gundam had conveyed from 1979 to 1993. The strongest mechanical devices in the known world are mobile suits known as 'Gundams'--absolutely, just like the Gundam series everyone loved before. The main character is torn between love for battle, his enemies, and the woman he loves--indeed, a classic protagonist of the Gundam world. And for one more example: The mobile suit which the protagonist uses from the start of his quest ultimately grows to match his own growth as a person and as a warrior--correct, reflecting one of the great metaphors of the Gundam series.
So while many reflexively look-down on G Gundam for being "un-Gundam" in its ways, with so much learned and reflected in its designs, calling the children of the Future Century to be disgraceful toward its parent would be foolish and unnecessarily harsh toward what is ultimately a truly great heir to the shining Gundanium throne. And with its beautiful joining of originality within the franchise and respect toward the parent, no Gundam series since its birth can match what G Gundam can offer. Take that, ignorant fools of reality, and join an era which truly extends itself as a Future Century of GUNDAM!