Both the Tremortusk and the Stormbird use shock-based weaponry to which all the other machines are resistant, so they're off the board for me.

But Stormbird has the aerial mobility, making it one of the biggest contenders.

Stormbird will just fly above and drop some lightning bombs and occasionally smash them with the ground attack. Most ranged weaponry is too inaccurate to hit it The other machines will eventually kill each other off, but none of them will be able to properly attack the Stormbird. The last surviving ground machine will have too little health to have a chance.

For ground based, I'd say the Slaughterspine wins, but then the Stormbird takes it out.

Yeah I have seen many youtube videos saying that the apex slaughter spine is the dangerous machine in the base game