Right after WotC bought 3E, Jonathan Tweet sent me a somewhat crytic e-mail. In it he asked, if I had the opportunity to publish licensed material for D&D, would I be interested? I gather a lot of people inside WotC/TSR thought that other RPG companies would not be interested, that we were too married to our individual games, that we scorned D&D as artistically empty or something. Of course, I love D&D, and started my career as a writer for D&D. And my answer was an enthusiastic, "Heck YES!" A few years later, we had this thing called the Open Game License, championed by Ryan Dancey. (And some companies did respond with scorn and conspiracy theories...and regretted it quickly enough.)
As others have noted, Jonathan did also write material for D&D prior to 3rd Edition. His name is not on as many 3E books as Monte, for certain, but then a lot of the names on that list haven't done anything (yet) for 3E that I'm aware of -- like Carl Sargent, Frank Mentzer, Tom Moldvay, and Dave Arneson (unless I've missed books they've recently put out, which is quite possible; I know Dave's working on one at least). And anyhow, the question asks "D&D author/designer," not "D20" or "D&D3E".
So, for whatever it's worth, and at the risk of disagreeing with Nicole, I think Jonathan belongs on the list.
Then again, the question is kind of vague and open-ended, and any list is going to be wildly arbitrary, given how many hundreds of D&D author/designers have contributed to the game over the decades. How about Kim Mohan, for example? Besides his authoring of some good stuff, his tenure as editor of Dragon helped shape the community of D&D players at the height of its popularity. Bruce Heard was mentioned; he not only wrote a bunch, but as the coordinator for out-of-house freelance writers, he was the gatekeeper of much of what TSR published for D&D for what, a decade? Roger Moore, as writer and editor, was always one of my favorites. Ken Rolston wrote some GREAT stuff, like the Viking culture sourcebook in the Gazetteer series.
Which one do I like the BEST? Man, I could not possibly decide.