Only one per category? That's annoying. I'll use "or" a lot because I'm lame like that.
Battle: Tiger, Lion or Wolf, depending on where I am the most. The tiger is agile and deadly in larger areas; the lion has the brawn for more close up fighting; the wolf has significantly more endurance and is a better tracker.
Stealth: For sneaking around, a domestic cat, or for assassinations, a Taipan. No one expects much from a tabby, especially in urban/suburban areas; the taipan is among the fastest and most venomous of all snakes.
Flight: Needletailed Swift (distance) or Golden Eagle (general purpose). The swift can fly the fastest in level flight of any bird, and for incredible distances (migrates from Siberia to Australia) - no duck for me! The Golden Eagle is the general apex bird of prey.