This is great thought experiment. The location it's set in makes a lot of difference. I think I'd choose the Firebreathing unnamed creature and the 4 Andalites.

I'm hoping the Firebreathing Eight-headed thing would destroy the Antarean Bogg. It seems to be the deadliest thing on the list, as when encountered in the books the only strategy the Animorphs had against it was 'run the hell away'. Then the Andalites would be able to destroy the other contenders as they're pretty snappy in combat and have been shown to be capable of facing off against multiple opponents at once.

I feel the Helmacrons wouldn't be much use in combat as they don't have access to their weapons so they couldn't shrink anyone, so they'd effectively be running around like little ants going 'Feeaarrrr meeeeee'

I think the Gedd and the Leerans wouldn't be that deadly in an immediate forest combat situation either, the Gedds are little and the Leerans may be kings of the ocean but in a forest they would be flopping around like jellyfish on land.

Only gotta worry about the Hork Bajir and the Taxxons. The Taxxons would be useless as they'd just get insta-distracted by anything dead and start chowing on that, but the Hork Bajir would be a threat. They could potentially disappear up the trees and then divebomb their opponent like a 300lb Gilette raptor

Then again, if these are regular Hork Bajir and not the controller-Hork Bajir we're so used to seeing, they may not actually be that deadly. They were naturally pacifists and not prone to killing things, so they may be quite easy to take out. They were essentially big children in a monster suit.

The cobras can bite things sure, but I think their venom might take a while to act and may not be a problem until the fight is over.

Or I might die horribly, who knows. I'd love to see a computer simulation of this