I think everyone will be sold on 'This book is the first.'
Some of my favorite 5e supplements were James Driscoll's free conversions of the 2e Complete Fighter's Handbook, Complete Priest's Handbook and Complete Thief's Handbook which he shared here on ENWorld. I felt his conversions were better done than the originals, because 2e lacked a system of feats (back in the day 2e days, I would have said advantages since that is what other games called them). That stated, I too found the originals inspiring.2e Fighter's Handbook was one of my all-time favourites - so inspirational.
As a kid I would read that handbook a lot and just imagine - an all-fighter campaign - Cavaliers, Gladiators, Myrmidons, Noble Warriors, Peasant Heroes, Savages...etc