Can I re-vote? I've just discovered something much, much worse than Tarrasque, Galactus or... well, anything.
Thirty. Headed. Tarrasque.
http://www.wizards.com/default.asp?x=dnd/eo/20070608a
Scroll down and gasp in disbelief. Almost 5000 hp. Caster level 106. Armor Class 69. Base attack 106, grip 142.
It can attack with 30 jaws AND 60 horns each round, dealing dozens of damage and a chance of a Devastating Critical Hit every time, which is lethal unless the victim saves the throw... And if all 90 attacks aim at the same guy, only a god or a demon lord stands a chance to survive. Slim chance.
Of course, you can't kill it normally, it regenerates limbs and heads, you must deal that 5000 damage AND cast a Wish or a Miracle spell to keep it dead. So even if those 100 soldiers make it somehow (...yeaaah, 50 soldiers with awesome equipment and flying horses would have a LOT of trouble with the normal, one-headed 800hp Tarrasque) and drop it to negative numbers, they still need a wizard to cast a spell before the monster regenerates. It means a Wizard must cast the spells in under a round, 6 seconds, or the monster wakes up again.
Author of this monstrosity said himself:
This creature speaks for itself. The multiheaded template allows it to exist, but it's still just plain wrong. The multiheaded template says this guy should have a CR 29, but I used the added Hit Dice to recalculate the CR instead. It seemed more realistic, and it's still a TPK machine. I thought about adding the half-fiend template to it, but then it becomes an instant-kill machine (as if it isn't already) because its blasphemy caster level is 106. This is probably the most powerful creature I can come up with, but I may surprise myself some day and outdo it.