It's difficult for me to tell, since I only have 40 D20 books or so. However, I have been relatively disapointed by two books:
- Relic & Rituals: Excalibur. There are interesting things in it, especially the description of the various races, that I would be prone to use in a pure D&D game instead of regular elves, dwarves, etc. There is also interesting feats and spells, and a few other things. Nonetheless, despite not being bad, the overall feel taste bland to me. In fact it just doesn't convey an Arthurian ambiance as I would have expected from this product. Don't get me wrong: this is not a bad product, it has many things usable; but overall it lacks a real Arthurian feel.
- Book of Exalted Deeds. I will soon sell this book on ebay. For me, how to say it, it's like the Christian Angelic mythology seen through a Walt Disney lense, plus terribly simple. This book has been a big disappointment to me.