May I decline a proposal from a man my parents have chosen?

Yes; marriage requires your consent, and declining for a reasonable cause is not disobedience.
The hadith of Khansa' binti Khidham shows that the Prophet ﷺ rejected a marriage forced on a woman who was unwilling. But consider your parents' views seriously, because they often see things their children overlook. The best way: listen to their reasons, state yours respectfully (for example, doubts about his religion or character, or your heart not being at peace after istikharah), and seek the view of a respected third party. Avoid declining merely because of looks or wealth if his religion and character are good. If the conflict drags on, consult a counsellor. See counselling.