"Pines of Rome No. 4. The Pines of the Appian Way" by Ottorino Respighi
I can’t say what my single favorite piece of Classical music would be… I hardly know about the genre. I actually think this might be from the Romantic period… not sure. Regardless, this last movement of Respighi’s Pines of Rome is astonishing. It was used during the flying whale sequence in Fantasia 2000 to great effect.