Monstrous Index (Monstrous Manual) - very fun large list of DnD creatures, such as the Gigantic Space Hamster (though I prefer the miniature variety).
My favorite (not necessarily D&D) Monster?
Faeries, closely followed by imps for similar reasons
Has anyone read or remember the Artemis Fowl book series? It taught me the valuable lesson that these pests aren't to be trifled with lightly, or you will live to regret it. That's what I love about them; non-adventurers understand perfectly well the damage that a swarm can cause to villages and crops, yet adventurers do not. As a DM, I would do all I could to truly represent faeries as something memorable. That does include tampering with adventurer rations.
For starters, just imagine "handling" a swarm of these guys, and they don't even have to be intelligent. Just try to guess which ones are poison while swinging that axe around.
Faerie, Petty, Bramble (Monstrous Manual)
Now imagine if they have access to spells. I'd consider this a template and allow some unseelie faeries to have the annoying seelie faerie spells that adventurer's wouldn't be able to handle easily.
Sprite, Seelie Faerie (Monstrous Manual)
Sprite, Unseelie Faerie (Monstrous Manual)
The possibilities grow when you mix the zombie undead template to the faeries (which I'd totally do as a rule of cool), and when you have faerie overlords, such as plague witches.