Do not forget: books are the creation of solitude and the children of silence.
Marcel Proust, Contre Sainte-Beuve
The Revised Players Guide for Masquerade, which comes at the end of a great run of “toolkit” books useful for either Vampire game or elsewhere, is really a better guide for Storytellers. It’s full of advice from multiple STs on a variety of topics. The bit on dealing with Lone Wolf characters alone should make it worth picking up. The section of playing mortals should too. The fact that it was the last hardback supplement before Gehenna is rather unfortunate timing.
The original Clanbook: Giovanni enriched them the most of all the clans, while the revised Clanbook: Ravnos reimagined them due to plot developments and made them a lot more interesting as a result.
Giovanni Chronicles IV: Nuova Malattia gets the prize for best adventure book as explained here.
I would count Blood And Smoke as the essential supplement for Requiem, but it supplants the rulebook so that makes it a strong contender for best edition instead.
The Danse Macabre road tests a number of its key ideas, so it’s effectively a players’ guide for the new book before it comes out. It also features ideas like street-level and global versions of the covenants and chronicle ideas designed to spin the game in different directions.
Speaking of different directions, Strange, Dead Love focuses on a major part of vampire fiction that Vampire tends not to deal with very much at the table.
And for an adventure, Into The Void / Reap The Whirlwind is pretty darn good, even before it was free.