The "d20" period of D&D is special to me because it brought me back to my first RPG love: D&D. I had long since replaced D&D with Call of Cthulhu, GURPS, Rifts, and any number of other RPG's. D&D3 "d20" reminded me of how much I liked the original D&D.
At first things seemed great, the core books wore solid, well thought out, and had some interesting innovations on the original system. But it didn't take long before the avalanche of utter garbage from the "d20 community" and the official published products themselves that I was overwhelmed, and sought refuge in those humble 3 booklets from the Original Edition.
Still, I have to give credit to "d20", it made me once again interested. And, while I don't have as much time these days to play, I still get to enjoy the occasional D&D game and exercise my imagination.
- D&D Player's Handbook 3.0. Don't knock this choice, it set the bar for d20 products to follow and opened the gates for fans and pros alike to start publishing d20 products of their own (and really, it was the start of the OSR if you think about it). Among the innovations I like are: removed the awkward "Advanced" moniker, just calling it "D&D" was a good move. Ascending AC, prestige classes, spells organized alphabetically (rather than by class), simplifying all the nasty things that can happen to a character into "Conditions", and removing sub-classes. Sure there was plenty to hate: Feats turned the game into a stat-block nightmare, attacks-of-opportunity were unintelligable, but overall it was a good refreshing of D&D.
- Tome of Horrors. This book felt as close to a 1st edition product as anything published during the d20 period. It brought back so many classics from the original Fiend Folio and was both nostalgic and contemporary - a difficult task to achieve.
- Arcana Unearthed. Monte Cook's magnum opus of a campaign setting. He pulled out all the stops to show what he thought the original Unearthed Arcana could have been, but wasn't. This book is well worth reading and drawing inspiration from. While I wouldn't play it as is, it certainly has provided inspiration for my own campaigns. (Actually, I want to say Arcana Evolved, because Monte Cook's revision of Arcana Unearthed was superior in almost every way. Really, a great book.)
- Call of Cthulhu d20. Of all the d20 books published by Wizards, this one struck me as the best. I love the original CoC and it is my choice of system when running a Cthulhu game, but for pulling Cthulu-esque elements into a D&D game the CoC d20 was the perfect resource. My only complaint is they got too clever with the layout - all the silly diagonal text layout was unnecessary and made it harder to read. But the quality of the writing and the design are superb.
- Delta Green d20. During the d20 heyday a lot of publishers would slap the d20 logo on anything, but Delta Green wasn't like that - it was a premium quality product.. The Delta Green supplement to the original Call of Cthulhu was one of the best written, well conceived RPG products ever designed. Pagan Publishing found a way to turn CoC into a sustainable campaign. It was creepy, brilliant, intriguing, and fun. The d20 version maintained all the quality of the original without compromises.
- Midnight. Fantasy Flight Games published a dark d20 setting with a simple concept: what if in the Lord of the Rings Sauron won? Midnight is a campaign world where the "dark lord" won the great battle in the war of good vs. evil, and now the heroes (the players) live in fear and are part of a rag tag rebellion, it is just a great idea. I might be biased because I illustrated the original edition, but the concept was so strong and the setting so well conceived it all added up to a great game.
- Mutants & Masterminds. The fact that Green Ronin continues to publish Mutants & Masterminds should tell you something. They found a way to take the d20 system and write, arguably, the best Superhero RPG out there. While Champions my be the, um, champion of "crunch", Mutants & Masterminds took the most familiar RPG system in the world - D&D - and turned it into one of the best superhero RPG systems you can imagine. I just love this game.
- Magical Medieval Society. Written as if by serious historians and scholars as if magic really existed during the Medieval period. This product might seem tedious to others, but it takes the proposition of magic seriously and the ramifications it would have on a medieval society. It is one of the most brilliantly written RPG supplements of any period, not just d20.
- Iron Kingdoms World Guide. Privateer Press was very smart in how it took advantage of the d20 open license. They published just a few EXTREMELY HIGH QUALITY (yes, the all-caps was worth it) products and sold enough of them to fund their true ambitions: a cutting edge miniatures company. The writing, art, design, and layout of Privateer Press demonstrate a dedication to craftsmanship that tempts me to call them the Apple of the pen & paper games industry. Everything they published for d20: Monsternomicon, Witch Blade Saga, and Iron Kingdoms deserve a place on this list, if only there was room.
- Pathfinder. Only because Paizo created something as close to the D&D Rules Cyclopedia for d20 as possible with all the power of modern graphics, game design, writing, and illustration they could muster. I just wish it wasn't in all in 9pt font, wall to wall text. But you still have to be impressed by that core book, it sums up just about everything and the quality is as good as it gets.
And that about finishes the top 10 products of the d20 era. Your mileage may vary.