A lot of factors go into making a monster book worthwhile for me. One is simply volume of monsters, how much bang am I getting for my buck. The other is how well the monsters are designed, are they balanced? Do they bring something interesting to the table mechanically? Are they something players have seen before in other media? This is really just the quantity/quality dynamic though, it’s not really informative to say “I want lots of well-made monsters”, because that’s what every DM wants.
I really like interesting/unique monster mechanics, which is a large portion of why I enjoy being DM and running them. It’s a great source of inspiration to see an unusual ability and contemplate how to incorporate and surprise players with it. If your group is new to D&D, then the brown dragon, for instance, is a great way to surprise them with an encounter focused on a burrowing dragon rather than one who more predictably wings onto the battlefield from afar. Forcing players to adapt their playstyles and expectations is what a diverse monster list should really do, and I think variety in this regard is strongly desirable.
Artwork is another big draw for me. It is what really stokes my imagination from a storytelling perspective, and good artwork can make me favorable towards even bland offerings mechanically. When good art and good design meet though, those are my favorite monsters of all.
As far as which ones I’ve enjoyed the most? That one is harder to say. In the short term it’s Tome of Beasts, easily, but overall it’s more muddled. I really enjoy books like Open Grave and Demonomicon, but unless you’re running a themed campaign then you only tend to dip your toes in the content. On the flipside, more general books of monsters tend to have more filler content and fewer pre-built customization options. At the end of the day 4e monster books with math fixes were my favorites. Good art, good mechanics, easy to throw together on the fly.
Utility-wise I think the basic Monster Manuals tend to win simply on general principle of having many generic offerings and stats for everyday critters. Setting those aside, probably the various draconomicons. Regardless of game dragons are usually expected to be a real show-stopper and those books are great for helping ensure that happens.