Hiya!
I almost said "Other", but then I re-thought about the question. Arneson 'created' the idea of a small group of 'heroes' (not as in "heroic", but as in "leaders of military forces as per Chainmail miniatures wargame), going on an 'adventure' into a dungeon/cave/ruin. But...to be honest...he didn't really "take it far". He was, iirc, more interested in the initial concept and as a sort of "well, we have several people that want to wargame, and only one table...lets do Dungeons & Dragons tonight".
Also, iirc, it was Gygax that really saw the potential and started really going whole-hog into fiddling and refining the rules, expanding them, and trying to solidify the concept of the game.
So that is why I give it to Gygax. Arneson was the spark and initial flame, but Gygax was the one that brought out the bellows.
Greenwood? Er...no. I mean, I'm sure he's a great guy and all that, I do like listening to him and what he has to say, and he does have a solid footing for "heroic stories of epic epicness" and all that, but, honestly, he just wrote a setting and teeny-boppers (re: 14 year olds, give or take) really latched onto all the over-the-top "heroes and villains". I see current Forgotten Realms as the "Marvel/DC Super-Heroes version of a fantasy world".
(and yes, I do have rules and have played in games where we use Marvel Super Heroes Advanced Set in a fantasy setting of my best friends creation called 'Urshurak'; and yes, before you ask, it's VERY fun!)
^_^
Paul L. Ming