Deities & Demigods 16
Fiend Folio 17
Monster Manual II 12 - for the Demons, Devils and Daemons alone, this book was wonderful - and then there was the Aboleth, Shades, Derro, more Hags, Barghests, Behir, Kraken, Gibbering Mouther, Yuan Ti, new Giants, and the THESSALHYDRA (vote up for Stranger Things!)
Unearthed Arcana 21