There really isn't any good fireworks viewing from any of the restaurants in the MK. CRT offers an interesting view from inside, but it's best if you've already seen them. From outside the MK, there are pretty good options at Narcoossee's (Grand Floridian) and California Grill (Contemporary).
For Epcot, the best viewing is from Rose & Crown (UK). There are some tables with a good view from La Hacienda (Mexico).
In all cases, none of the restaurants will guarantee a table with a view. Sometimes they will let you request one, but even they will not guarantee you a table in time for the show. Luckily, some have alternative options (e.g., Narcoossee's has a veranda, Cali Grill has an observation deck, and Rose & Crown has a reserved viewing area).
The view from the Veranda at Narcoossee's:
The view from the observation deck at Cali Grill:
The view of IllumiNations from the reserved viewing area at the Rose & Crown:
The view from inside CRT: