It's the Magic Kingdom (Wishes) fireworks which you get the best view of, in the distance across the lake. Epcot is actually behind the boardwalk shops and you can see bits of the Epcot display across the top of the Epcot Eiffel Tower. To do this you need to turn right when you enter the boardwalk and walk along to where the shops end - I think the last shop is the ESPN one. There's a 'shoot the basketball' type stand and just past there is your best bet. One more thing, unlike Downtown Disney, they do ask the driver for a photo ID before letting you in the boardwalk parking area so make sure you have your driving licence with you (it's the law anyway!)