Top three, here we go...
The Complete Fighter's Handbook
2nd Edition
1989
Right after they released the 2nd Edition rules that just codified the 1st Edition rules in one place and streamlined some stuff, they added major ideas to the game. This book became essential. Not only did it add things like new weapons to the mix, but it also created a lot of new ideas for ways that characters can fight. Specialization in fighting styles was brand new, as well and broad and narrow group proficiencies. However, what the CFH will be forever known as was the introduction of Kits to AD&D, and it was one of the best additions to the game from 2nd Edition.