Dread Blossom Swarm (MM3). If swarm damage that causes nausea wasn't enough... they exude a cloud of poison that paralyzes and then suck out your blood with Con damage. And if being immune to weapon damage wasn't annoying enough, slap Regeneration 5 on top of that. Another cruel trick: put these in a "garden" with a Living Spell (same book) such as a Glitterfire or Sickening Sleep. Add the Sentry Ooze template (Dungeonscape) and the Thick-Skinned feat (Savage Species) so it's immune to the swarm damage.

Sporebat (Fiend Folio). It's a plant that looks like a spawn of Cthulhu... that flies around and spams enervation death beam lasers every other round. Oh, and it has 15 HD for some reason.

Fang Golem (MM4). They explode when they die, so put something breakable around them, like Porcupine Cacti (Sandstorm) or Fetid Fungi (MM5). Gas Spore (Lords of Madness), Bladerager Troll (MM5), Phoelarch (MM3), or Chraal (MM3) also have "Death Throes" explosions.

Hoard Scarab Swarm (Draconomicon). Not only is it treasure that attacks you, they burrow into your skin (like in that Mummy movie) and deal Con damage until you're dead. If you don't have a remove disease or heal spell handy, you could be looking at a permanent dirt nap.