Actually, this is quite tough
Geek that I am, this is like voting for the best Trek series. I am firmly a product of today and continuous storylines, so I wouldn't vote for TOS but it would certainly get honorable mention. Also, as a vote for the moment, I can only talk about my current thoughts and influences.
As Alternity is my favorite game system and I am playing it, I would go with Bill Slaviscek, Rich Baker, JD Wilker, Sean K Reynolds, Monte Cook and Andy Collins. Further, JD has been hanging out at alternity.net and has been very cool to read his opinions on things.
However, CJ Carella has really impressed and influenced me with the Unisystem, via Buffy, Angel and Witchcraft. His writing is top notch and interesting to read and the style fits my own style of play, with the emphasis on the characters and drama. I also give him top ranks for being the first person that I have read to put game mechanic contraints on the GM that make perfect sense. (To contrast this, I am all for diceless but allowing the GM to lie and cheat, as Amber did or any system, doesn't work for me. And it is handled so elegantly with regards to drama and the characters in the Unisystem mechanics.)
The people at White Wolf have also influenced me a lot. I was buying and reading Vampire, but not playing, because their Dark Ages stuff was a great fluff resource for when I was playing DND and now in my (fantasy) Alternity campaign. While I find the rules a bit clunky, the ideas behind them as well as the very cool settings, have made all of that material very good and it has certainly influenced me. While I don't like the anime style of Exalted, I would love the opportunity to play it at some point because I think the game play would be lots of fun!
Joe G Browning and Suzi Yee over at Expeditious Retreat Press are putting out some excellent, almost generic, fantasy supplements that are top notch. The MMS line is great and I look forward to their take on the Romans as well as more historical lines that they have talked about doing.
I stopped playing DND/d20 about the time that many very good authors appeared. Many of the EN world publishing have been good and usable in things beyond d20. (Toxicants, In The Saddle, Taverner's Trusty Tome) There are really too many to name and I don't know all of them by name, unfortunately.
Troll Lord Games EGG series, some of which weren't penned by EGG, are also very good for someone like me who is looking for some good, generic, fantasy products.
Since I started playing DND a long time ago, I also agree with the list of David "Zeb" Cook, Roger Moore, Kim Mohan, Ed Greenwood, Steven E. Schend, Jim Butler, Colin McComb, Jeff Grub, Dale Donovan, slade, Julia Martin, Elaine Cunningham, Eric L. Boyd, Skip Williams, David Eckleberry, and I am sure I missing many others.
The people at AEG for Stargate and Spycraft, which I haven't played but have really impressed me with their design and conversion for the d20 rules.
Again, the designers of d20 Modern, Charles Ryan, et. al., get mentioned as well because I wish DND had gone to something like this. I don't have it but I am excited about the read that will be d20 Future and probably d20 Past.
As with Roddenberry, honorable mention to EGG and Arneson.
Unfortunately, I am not playing any d20 right now. I am a firm believer that the underlying game mechanics influence and set the style and tone of play and until d20 moves away from its wargaming roots, I don't see me playing any d20 for a (long term) campaign. Having said that, there is a lot of good material that I will gladly buy and convert it to another system, such as I have done and want to do in the case of Stargate to Alternity. I do mostly enjoy d20 for one shots, such as when my old gaming group gets together.
Great discussion! Thanks!
Have a good one! Take care!
edg