Image from paizo
5. Pathfinder Bestiary (Paizo, 2009): Paizo may have saved our hobby from the 4e massacre. They picked up the 3.X mantle and gave the players something worth playing. Paizo put a great monster book together here, with amazing art, and a sturdy frame. I did enjoy Pathfinder for several years, but now I prefer less "crunchy" games.
4. AD&D 2e Monstrous Compendium (TSR, 1989): I loved this book. Right up until I bought my 4th or 5th Volume. I loved that I could arrange the monsters in my notebook next to the encounter descriptions. I loved being able to keep hundreds of monsters in a single binder. Then the holes started ripping. The printers couldn't line up the pages from volume to volume. I spent way too much time putting them back in order when I was done. It will always have a place in my 14 year old heart, and on my shelf.
Image from Troll Lord Games
3. Castles and Crusades Monsters & Treasure (Troll Lord Games, 2004?): I love C&C. It's probably my favorite game behind Swords and Wizardry. I rank this book because I only need it for, well, monsters and treasure. C&C doesn't suffer from too much crunch. And Troll Lord seems to always be running a sale.
Image from Drive Thru RPG
2. Swords and Wizardry Monstrosities (Frog God Games, 2013?): As much as I love S&W, that's not why this book makes #2. What makes this book head and shoulders above the modern competition is that every monster has an example set up. At the very least, this makes it entertaining to read. And, more often than not, I want to find a way to included the described encounters into my game. To top it off, it is a Frog God production, so the book will probably outlive the campaign it is used in.
Image from Tome of Treasures
1. Monster Manual (TSR, 1977): There's not much to say about this book that you don't already know. We all love it. A book with the same name has appeared in 4 of the 5 D&D editions (sorry 2e). This book has stoked imagination in one form or another for 2 full generations. Its longevity and iconic nature make it a must have, in at least one edition.
A quick note: I wouldn't trust those prices at Tome of Treasures. They seem a bit out of date.