Well, according to
http://www.enworld.org/top20.asp, the top 10 D20 items are:
1. Manual of the Planes (WotC)
2. Spycraft (AEG)
3. Starfarer's Handbook (FFG)
4. Wild Spellcraft (Nat20, personally, I think it's over-rated; the author could've done a much better job)
5. Four-Color to Fantasy (Nat20, and though it is kinda flawed, it's the most flexible d20 super-power supplement; so good, you can even use it in fantasy games)
6. The Legion of Lost Souls (Privateer)
7. The Book of Eldritch Might (Malhavoc)
8. Player's Handbook (WotC)
9. What Evil Lurks (Necromancer)
10. Oriental Adventures (WotC)
11. Counter Collection II (FDP, which I'm only listing because I have a small writing credit in it).
Judging from this, the top non-WotC publisher is probably Natural 20 Press, though again I say the reviews are over-rating their products. Maybe you should review them too and lower the scores accordingly?