That's a dangerous question to ask a zoology nerd.
So I guess I'll start with some of my favorite extant chordates.
Favorite terrestrial mammal: Gorilla
Probably at least somewhat owing to my love of vintage sci-fi and horror films, a bizarrely large amount of which feature gorillas.
Favorite aquatic mammal: Manatee
They're just these big adorable floating balls, and from what I hear they like belly scratches.
Favorite reptile: tied, American Alligator and Alligator Snapping Turtle
The both of 'em are just the huge primeval monsters, and it's hella impressive.
Although they are both overruled by any turtle with two heads.
Favorite bony fish: Coelacanth
Reasoning: See reptiles.
Favorite cartilaginous fish: Hammerhead Shark
I just like how they always look like they're saying "aww, c'mon that wasn't very nice."
Favorite jawless fish: Hagfish (aka slime eel)
Once again, weird primeval beast.
Okay that's good for now I'll come back for the other phyla.