Goodness, but there are a lot of answers to this question.
The absolute best place to view IllumiNations (the fireworks at Epcot) is from the deck of your private pontoon boat. The specialty cruises are $120 for up to 12 passengers (or, they were that price the last time I was in the World, anyway), and need to be booked about .2 seconds after 7:00am Eastern 90 days before you want to cruise... I think it's the WDW-PLAY number, option 4, for cruises.
The absolute best place to view Fantasy in the Sky (the fireworks at Magic Kingdom) is from over your meal at one of the two tables in the California Grille that are located up against the big catty-cornered window that faces the Magic Kingdom. In December and January, it gets dark early enough that you can get one of the opening-time 5:00pm seatings, show up early and request one of the "prime" tables, and have the fireworks go off right in front of you about the time your (ordered in advance, of couse) dessert souffle arrives. Other times of year, getting a prime table at perfect fireworks time is much more of a crap shoot.
If you want to get prime seating without the big expense, you may have a trade-off in "camping out" time. For IllumiNations, I recommend the Mexican Cantina. For Fantasy in the Sky, I recommend the water-front seating areas in front of the Plaza Restaurant and Plaza Pavilion Food Windows. Tinkerbell flies from the Castle to the overhanging roof on the Plaza Pavilion seating area... this is the closest you'll get to the fairy at WDW.
If you're looking for a "last minute" place that you're still likely to get a good view, I can't help you on IllumiNations... too much going on at water level that gets blocked if you're behind folks. For Fantasy in the Sky, the already mentioned Tomorrowland Bridge works for Tink sighting, in front of the Crystal Palace works for fireworks-and-Castle-in-the-same-viewfinder needs, Toontown is the closest point to where they're actually shot off, if sound effects are part of your fireworks experience, that might be best.
And as far as MK fireworks go, I can also tell you from experience that even if you'd decided to blow them off for that night, and you're walking through Frontierland away from the Castle when you hear them, and you pause and turn around for a quick peek...
...it's still a pretty good view.
