Start with Mozart, move to Beethoven. Wrapping your head around classical styles is easier when you directly compare the Classical and Romantic styles since they are quite different. That gives you a foundation to understand other forms.
Palestrina is an interesting suggestion. That makes sense from a perspective of chronology and the development of the form, but Bach and Mozart have a big advantage of familiarity. I've heard Palestrina's music in university classes, but I can't recall any of it at the moment.
