There is no one spot that's best. There are a number of good options. Where are you staying? Which fireworks? What date? If you are going to Universal, then transferring to WDW in the evening, your day will not be wasted. There are actually a number of good places to eat at Universal.
What kind of dinner do you want? what price range?
On low cost option, s to get food at any quick serve place inside either MK or Epcot, then head to almost any location on the World Showcase lagoon inside Epcot. The only bad spots are those blocked by trees, and those that are downwind.
For MK, it is generally understood that viewing posts in front of the Castle offer a great view, but you'll also be in a throng of people, and you'll need to allow plenty of time to get a spot.
HS has an evening show, but no fireworks. AK does not have any fireworks, nor is it open very late most days.
At kind of the far end of the spectrum...you could make dinner plans at Narcoosee's or California Grill. They are both high end dining options (and require appropriate attire), but both offer great views of the fireworks. At both locations, if you dinner is over before the show starts, you are allowed to view the fireworks from a deck or platform.
Also, some of the resorts offer fireworks views. One offsite option is the Marriott World Center. not all rooms have fireworks views, but many do. A high floor is good. Some onsite rooms also have views of the fireworks. that said, if your room has a view of the fireworks, then you will probably already know, because most come at a premium price. (MK view or Epcot view) One last resort option that is kind of offsite and kind of onsite are the Swan and Dolphin. not all rooms have a view. You will know yours does because you will be paying for it, and it will be listed in your room rate.
The Epcot fireworks are almost always at 9pm, MK fireworks times vary.
Last, there are a few out of the parks, non restaurant viewing options, but they all have some limitations. You will miss out on the audio, have a partly blocked view or maybe have limited free transportation to your resort after the fireworks. (depends where you are staying)
A little more info would help.