Monster Vault and Monster Vault: Threats of the Nentir Vale (4e D&D)
These probably won't make anybody else's lists. They were late additions to 4th edition towards the end of its run. They didn't really add anything new or innovative to the system. Instead, they offered a "corrected", more simplified approach to the game system's maths and mechanics, which was part of the reconfiguration process introduced by the Essentials products. Regardless, these two items still had a lot to offer.
For starters, each boxed set came with a complete set of full-color, double-sided tokens for every monster and npc listed in their respective books. Considering the emphasis of using minis and maps for 4e, it was surprising (disappointing?) that this wasn't done more often. To my knowledge, no other "monster" book has offered this as part of the original product. (e.g. Paizo offers pawns for each book as an additional purchase, not part of one). I could be wrong, but not for any official DnD product, now or then.
The content itself was pretty standard fare. Hundreds of entries for commonly used monsters and adversaries you might encounter in a typical D&D setting. In fact, there were very few new or original concepts included in these books. What was unique, however, was seeing multiple entries of variant monsters with different roles and designs that complimented each other, providing new ways to surprise even the most jaded players with unpredictable encounters of familiar monsters.
For example, goblins no longer had stock abilities or stats. Stealthy blackblades complimented brutish skullcleavers who defended magic hexers while minions swarmed their enemies. Each had specific strengths and weaknesses, even though they were all goblins.
Threats of Nentir Vale took things a step further by creating unique organizations and personalities for their default sandbox setting. In essence, the book was as much a campaign toolbox for DMs wanting to run games within that area. It was such a fine balance of crunch and fluff.
