I'm not sure from your question whether or not you know what fireworks are on offer.
Wishes is at MK every night, except on some special events nights, such as Mickey's Not So Scary Halloween Party when they show a different version (HalloWishes), On such evenings, you have to have paid for the party to see them from within the park. The timing of Wishes varies throughout the year.
IllumiNations, Reflections of Earth is the current nightly show at Epcot. I can only remember one very short spell when it wasn't shown at 9pm (and then it was 9.30pm).
Fantasmic is the nightly spectacular at MGM Studios, but it isn't a fireworks show. There are no fireworks at Animal Kingdom for obvious reasons.
As you've probably gathered, both Wishes and IllumiNations tend to garner fans in equal measure. I prefer IllumiNations, but I think it's more of an acquired taste than Wishes. I'm convinced that the music is a big part of the appeal of parades and fireworks shows and, now that I'm familiar with it, the Reflections of Earth music is one of my favourite pieces. Like Tim, watching IllumiNations never fails to move me. However, there is an extended period in the middle where there are no fireworks and, unless you have a very good view of what's happening on the lagoon, it can be a bit boring, especially for children. Reading up on the fascinating facts about the globe (which is where all the non-fireworks activity takes place) might help you to appreciate it more.
Wishes is more accessible "straight out of the bag". It's much more your regular fireworks display, although, being Disney, there are some pretty spectacular fireworks effects. Tinkerbell starts the show by making a flight from the top of the castle, but you need to be reasonably close to see her properly. It's possibly the better choice if you have young children.
Whichever you choose, your viewing location can make a big difference to your enjoyment. Make sure there is nothing to block your view, particularly trees.
At MK, anywhere down the middle of Main Street will be fine. The beauty of this vantage point is that the fireworks are designed to form a symmetry over the castle, so you get the full effect and, no matter how packed it is, you'll be able to see reasonably well because you are looking skywards. If it's really busy, you may have to lift small children. If you want to watch from the station platform, you'll have to stake out your place well in advance - possibly up to 2 hours. There are some good viewing spots to the right of the castle (the Rose Garden and the bridge to Tomorrowland, for example) where you'll get the best view of Tinkerbell's flight. Be careful if you pick a spot on the other side of the Castle because there are a lot of trees which can really spoil things. If you watch from behind the castle, you won't get the proper effect because the fireworks are launched from behind to appear as though they're above.
At Epcot, there are lots of great viewing spots. Go on a quiet night and you won't have to arrive too early to get a spot right on the railings. Before it gets dark, take a look at where the globe is located on the lagoon (they take it out mid afternoon) - some popular viewing locations (such as the bridge between UK and France) are much further from the action than others and the globe segment has much more impact from close quarters. Again, take care to avoid trees and other tall structures which could block your view.