I agree with ahutton for the MK (Fantasy in the Sky) fireworks, I like the Pizza Pavilion. You COULD get a table at the California Grill. It is hit or miss with regards to getting a window table, but restaurant guests can go out to the observation deck, if it is open.
The Rose and Crown is a great place to see the fireworks in Epcot (Illuminations). This is also hit or miss as to whether you get an outside patio table. They have been inconsistent with whether they will accomodate seating requests or not. It may depend on who is running the floor on a given night.
For a more casual seating, I like one of the tables in the Cantina in Mexico. There is one right near what looks like an old boat that is washed up on shore. That is a speaker for the music. During busy times, we have seen those tables fill almost 2 hours before the show.
The fireworks at the Studios (Fantasmic), as stated, are a bit different. That is a show that is in an arena. You can bring in food, or buy popcorn and drinks from roving merchants.
Ted