Definitely 3rd Edition. I don't know if it was because of the nature of the game itself, or simply the ability to discover and discuss it online suddenly coming to the forefront. It was the first WotC edition, the first edition that really involved fans at all, given the voracious appetite for information online, and it was the first edition to actively encourage the fans to make their own stuff, to even publish their own stuff and share it with the world.
My memory of 3e is also of the 'community' being the strongest, but I have to mention that with a cautionary note- 3e was also the first edition where there was a 'the community'- prior to 3e, we had a lot of little, balkanized communities because the internet was not yet a mass phenomenon. Yes, I played in 2e PbP games and swapped house rules and stats online, but it was very limited in scope- I was a Prodigy subscriber, and what was going on on AOL or Compuserve or among university folks who had regular Usenet/Gopher/whatever access might as well have been happening on another planet. From that perspective, I don't know that we can recapture the feeling that accompanied 3e; ironically, the OGL and everything that took place with Pathfinder and 4e means that now there is much less a sense of there being a single community or a single center for that community (WotC's own boards, for instance, feel much more impoverished today than they did during the high water mark for 3e in terms of the volume of posting, number of subscribers, etc.)
I think Wizards needs to accept that the community has spread out beyond what a single site or a single focus can encompass- encourage OSR and Pathfinder and everything else that can, by squinting right, be called D&D. Build that as a D&D-derived ecosystem rather than trying to consolidate. Be very liberal about fan sites and highlight good fan materials and discussions that are being posted on places that aren't wizards.com. For instance, it would be great if they would do cross promotion with Paizo, or highlight people using 5e or 3e rules to create and run old school content (like Dwimmermount).
I kind of doubt that this would happen, as it runs counter to the received wisdom regarding how to create a 'web portal' for a brand, but I think we are at a point where a lot of the community is beyond needing that, and if 5e is going to be as embracing of multiple styles of play as Wizards claims, there is going to need to be some differentiation. In an ideal world for me, Wizards runs a community site focused on D&D/RPG 101, and then highlights cool 3rd party sites that are focused on particular aspects of the game or playstyles. I already have communities or sites that I read for specific things that I am already interested in that I don't see myself abandoning in favor of a 'new' Wizards 5e community, but on the other hand I would check the Wizards D&D site often if I thought that the chances were good that it was through them I would find something new and exciting that changes the way I see the game, like Pathfinder or OSR.