Leiningen, I didn’t know you posted here.
I say ants too. The only way to defend against them would be to wear some sort of armor. Something that could cover the body and stop their bites. If only humans could invent such a thing…
Does a single-celled plant count? If so, I’m voting for caulerpa taxifolia. There are some that fear that it is already choking off the oceans, and that could very well cause mass extinctions.
I’m hoping that the hype is overblown, in which case I’m voting for sharks with freakin’ laser beams on their heads.
It’s beyond my academic depth to evaluate, but I do see quite a few Google hits on something called “gut-origin sepsis.” And even if sepsis were out, isn’t it pretty well accepted now that there’s a bacterial origin for some kinds of cancers? There are ten times as many gut flora microbes inside you as there are “your own” cells. If they decided to get up to something, I’m betting the results wouldn’t be pretty.
Heck, they could decide to spontaneously evacuate themselves and a heck of a lot of people would die just from that.
Your gut flora is already trying to kill you. So is almost any kind of bacteria or virus that’s potentially harmful to humans. These things have partially fulfilled the premise of the OP in that they already do everything they can to kill people. Not out of malice or even choice, of course, they’re just feeding/reproducing and it so happens that we’re good hosts for them to do it. But generally they’re stymied by our immune system, medicine, sanitation, and hygiene. Maybe one day one of them will completely ‘beat us’ but so far they haven’t managed it.
I think we can be a little flexible with the intelligence thing. My main reason for including that caveat is to prevent super intelligent animals with human-like abilities for reason from being considered. If you say a million smart chimps wanted to destroy humanity, you may as well say a million humans suddenly turn against their own kind. With our ingenuity, there’s no scenario that would be completely impossible if we use a little imagination
Locusts, while I don’t think they should work together, can still do what they normally do, which is eat. Suppose they gravitate towards our food supply somehow. I wouldn’t call that strategizing unless locusts from 2 different areas conspire to destroy crops at the exact same time. Play with the premise however you want
You know, I totally forgot about rats. I think they could come close with some luck on their side. They did it before, but admitted that was with humans at a far less technological standpoint
That’s why I say that once you die, in a certain sense you’re more alive than ever.
You know, it occurs to me that it might not be the brightest idea to volunteer these kinds of ideas to someone named “YogSosoth.”
Cats.
Because they already have, aeons past. They vanquished humanity and forced us to provide them with shelter, food and tummy rubs.
Their oppression has been going on for so long that they have convinced us it is they who are our pets.
If you want a picture of the future, imagine a paw stamping on a human face—forever.
Terraplane has it right. You are ascribing abilities to bacteria they simply don’t have. They can’t go maching out of your colon on their little legs, because they don’t have any. Some bacteria have flagella, allowing them some freedom of movement, but they don’t have the ability to navigate through your body at will. They don’t have nervous systems. All they can do, they already do.
Well, at least he isn’t Nyarlathotep.
Mosquitoes. Hungry mosquitoes.
Hippos. Hungry hungry hippos.
Um, fish *already *swim away from predators…
I dunno, since gut flora are arguably in a symbiotic relationship with humans, I’m not sure they’re doing all they can to destroy us. What if they somehow changed their genes to promote active parasitism rather than symbiosis? Or simply self destructed: they wouldn’t need little legs to evacuate the gut, just the ability to explode their little organelles. Is that any more far fetched than, say, all the army ants in the world suddenly overcome with bloodlust and a mission of “destroy all humans?”
Bats. If they’re prepared.
It’s funny how layfolks’ sense of taxonomy works: We’re restricted to one “kind” of animal, and I’m sure nobody would accept, say, dogs and cats (the same order) as the same “kind”. And yet, the OP explicitly allows “snakes” and “spiders” (both also orders) as valid “kinds”, and we have folks coming in with suggestions of “bacteria”, which are an entire Domain (the level above kingdom), or even two Domains if they’re lumping the Archae and Bacteria together.
And I also think the OP’s definition of “all-out war” is misguided. There are plenty of animals that humans have been trying, and failing, to eliminate for millennia. But none of them are trying to eliminate us, nor would any of them start trying without a significant change in intelligence. They’re not trying to eliminate us, but to conquer us. What rats want is to eat our grain, see us driven before them, and hear the eeks of our women. And so far, they’re winning.
Gut flora doesn’t have the ability to penetrate the protective mucosa lining of the gut. You’d have to grant it access somehow, like give it the DNA to make a digestive enzyme that could eat through our intestinal wall.
If I might be so presumptuous, I’d propose changing the OP a bit to introduce an alien intelligence determined to use some limited category of creatures (not necessarily “animals” per se) to destroy humans. The animals themselves are not changed physically or intellectually, but they are driven by a malevolent force that will direct them to do whatever is physically possible to rid the world of pesky humans, even at the cost of the creatures’ own lives.
Of course under these rules, I win. 
ETA: Or maybe not. Going by what WhyNot says above, I guess penetrating the intestines is out if we assume no physical change. But how badly would it hurt you if all your gut flora spontaneously self-destructed? Would it be possible for them to start churning out poison instead of nutrients?
Any symbiotic relationship they have with us is symbiotic only because our bodies force it to be so by creating a number of defenses to keep the little critters from devouring us. If they get a chance to eat your intestines from the inside out they’ll take it. All they really know is to eat, excrete, and multiply and if they have food available they can’t choose not to eat it. If that food is you then so be it.
Also, as far as changing genes, no it’s not really more far fetched than army ants on a mission of genocide but I don’t think it’s in the spirit of the OP.
Also also, I just saw your latest post. They already churn out toxins. Our bodies are just adapted to deal with them.