Dog, a Chihuahua to be specific.

I could probably hold my own against a tardigrade.

I will take on every animal at once, and win.

By being elected president on a platform of bog-standard normal liberalism, FDR style, behind a remotely charismatic personality rather than a shambling horrid human corpse. I will legislate the space force to create huge satellites that catch solar energy and funnel that energy down to the surface with big microwaves. I will take this opportunity to equip the space stations with hypersonic aircraft that will drop normal supersonic personnel carriers, ensuring a global response time of only a few hours. This will probably be less monetarily intensive than putting a US military base everywhere on the planet, so I'd use those savings to expand the nuclear arsenal, and possibly deploy some of those weapons to space in secret under the guise of some commercial wi-fi satellite ventures. I will reveal this fact to everyone later on once they have all been globally deployed and nobody has any countermeasures, and then I'll start performing a bloody hostile takeover of the planet.

Then, I will attempt to quintuple global fossil fuel output. I don't know what we'll use all this excess energy for, probably we'd just use it to build more horrible weapons of war, or huge impenetrable underground citadels, or whatever. I will get rid of regulation for industry, ensuring massive environmental disasters. I will even tell the CIA to do some of them probably, nord stream pipeline style, and they'll probably do it cause they're crazy. Maybe I'll use the microwave power grid to blow up some of my enemies by boiling them until they explode.

At the end of my term as god emperor dictator, a disgrace and shell of my former self, I will use the nuclear football to ensure no life on the planet survives, except for maybe basic viruses, bacteria, and maybe a couple different insects. I will arise from my presidential super-bunker to face a barren world. A perfect world, free from sin. Thus concludes the 2nd Global Emu War.

If I wasn't going to do any of that and I just had to give like the least dangerous animal I personally could take on, I'd probably say like. Maybe a stray ant. That might be too sad, though, because that's just a lonely ant and it's sort of too pathetic to kill it. Maybe like a really evil guy that's about to die anyways? But that's also too sad, because that's just a meat-puppet automaton of life that has shambled around until it's shut down. Maybe I could just kill like, dick cheney, or something, someone super evil. He looks too much like george costanza for me to do that though, I think.

Edit: actually I think I could take on any invasive species of animal barehanded, with a combination of my extremely tough fists that I have been spraying with dog medicine, and tai chi exercise DVD training regimen.

Hand to hand, with no weapons and just my bare hands & teeth? I could maybe take a mouse... and I'd still probably come away with some wounds. Probably a small, non-venomous snake as well.