Here are some of the places considered to be the best spots for viewing the parades & fireworks-

Magic Kingdom - Share a Dream Come True Parade

Main Street is a good spot with the castle in the background, as is the centre of Town Square.
Good spots can be found in Liberty Square, where it won't be as crowded as Main Street.
In front of the stocks in Liberty Square.
Grab a spot in front of City Hall or Tony's in Town Square, which is often uncrowded, and offers a great view of the parade.
Parade runs from Frontierland to Main Street, U.S.A.
The parade floats stop at intervals to allow guest participation, for details of where these stops are, see the Share a Dream Come True page.
"We watched the Share a Dream Come True parade from the porch of Liberty Tree Tavern. Not right in front but over more towards Frontier Land. It was a great spot."

Spectromagic

If there are two parades, watch the second one, it should be less crowded.
The most popular spots are along Main Street.  
A good view if you are in a wheelchair is at the hub, looking directly down Main Street, but get there early as is fills up quickly.
On the covered walkway between the Diamond Horseshoe & Liberty Tree Tavern.
One of the best views is the very centre of the platform of the Walt Disney World Railroad depot.
In front of Pecos Bill's.
On the bridge near Splash Mountain.
Get a table by the rail at Sleepy Hollow in Liberty Square (only if the parade is coming from Main Street).
On the platform in front of the Frontier Trading Post.
Last minute spots can usually be found in Liberty Square or Frontierland.
Note - if there are two parades in a night the first will go from Town Square up Main Street towards the castle, & the second will come back down Main Street.
Take your place 1-2 hrs before parade start time to ensure a good spot.

"Wishes" fireworks

Main Street, with the castle in the background. Don't stand too close to the castle as your view of the fireworks will be blocked.
The bridges between the hub & Tomorrowland & between the hub & Liberty Square, although in these two locations the fireworks won't be behind the castle, they'll be to the side of it.  If you are on the bridge to Tomorrowland, Tinkerbell will fly right over you!
 If you're watching the fireworks at Christmas time, & you are on Main Street, the garlands across the street partly block the view of the fireworks.  
Other good viewing spots are the viewing area on the 4th floor of the Contemporary resort, & California Grill restaurant at the Contemporary. Guests used to be able to watch from the observation deck on the 15th floor, but that is now restricted to those eating at the California Grill.
On the beach at the Polynesian or Grand Floridian.
"I have been to Disney World more than 17 times(and i am only 15). And take it from me, a great place to watch Fantasy in the Sky at Magic Kingdom is in between Mickey toontown fair tent and the railroad, up in an alley. Last time my family went Winnie the Pooh(there is a cast area back there), came out and watch the fireworks with us." Liz
A great place to watch Wishes and avoid a shoulder to shoulder crowd is in front of Crystal Palace. If you are lucky to catch a bench, it's even better. Although, you'll have to sit backwards" - Reg

Disney's Hollywood Studios - Disney Stars and Motor Cars parade

Near Indiana Jones.
On Hollywood Boulevard.
Between Star Tours and Indiana Jones.
The parade starts off at the Star Tours gate, travels down Hollywood Boulevard and exits near the front of the park at Crossroads.

IllumiNations

For a good view of the show you need to see both the centre of the lagoon & the sky.
The bridge between France & the UK.
On the second floor deck of the Japan pavilion, next to the Mitsukoshi restaurant, also has a good view of Tapestry. To get there, go up the stairs near the Mitsukoshi Department Store.
On the Friendship boat dock or waterfront in Germany.
In Canada, there are two benches on the waterfront between two food carts, opposite the totem poles, get a seat about 2 hrs in advance & you'll have a great view of the show.
On the waterfront in Italy, near the gondolas.
World Showcase plaza is a great viewing spot, but you need to get there early.
The area in front of Italy has a good view.
Book priority seating for the Rose & Crown for about 7:30pm & when you book in ask for a table outside. If you aren't able to get an outside table, you will still be able to go outside and watch when it starts.
There is a small park behind the Rose & Crown pub that offers a great view of the show, but it may be closed off for private parties. Walk past the pub towards France, just after the Yorkshire County Fish Shop turn off to your left.
Get a seat on the veranda of Cantina de San Angel in Mexico.
For the very best seats you need to arrive at least 90 minutes early, some go two hours before.
Wherever you choose to stand make sure the wind is blowing away from you, otherwise you won't be able to see the fireworks for smoke!
"My favorite spot for IllumiNations is right in front of the monorail track & Spaceship Earth (the ball) It's very easy to get out of the park when it's over, so you don't run into anyone."

Animal Kingdom - Mickey's Jammin' Jungle Parade

Starts at the Tusker House Gate at Harambe Village. It winds round Discovery Island, and returns to the Tusker House Gate.
The path between Africa and Asia.
Between the bridge to Camp Minnie-Mickey & the bridge to Africa.

Fantasmic

The best views of Fantasmic are anywhere in the central area, especially at the front, although be prepared to get wet (a fine mist comes from the water screens on the lagoon, & by the end of the show you will find you are quite wet!).
 In order to get a good seat you will need to start queueing about 2 hrs prior to show time, usually 30 minutes before showtime there is standing room only.
A Fantasmic lunch or dinner package is available, which includes lunch or dinner at Mama Melrose, Hollywood and Vine or the Brown Derby, & reserved seating at Fantasmic. Sometimes the package is also available at 50s Prime time Cafe, & Sci-Fi Drive-in, so check when you book. The reserved section for Fantasmic is on the right-hand side of the theatre, & you must turn up 30 minutes prior to showtime. More info...

Comments from other visitors

"outside christmas shop magic kingdom.anywhere in frontierland for all m.k parades.mgm between star wars and indie!"

"We got to Spectromagic over an hour before it started. We went to Frontierland and found a great spot. If you are standing in front of the Countrybear jamboree there are some storefronts to the left. On the one there were 3 rocking chairs. My wife, daughter and I made ourselves and home and watched the entire parade from the chairs. Sure beat standing or sitting on the cement."

"Our favorite spot to view the 3o'clock parade is in front of the Emporium on Main Street. We usually arrive at least one hour before the parade and secure on seats. Then, someone is designated to get cold drinks from the seat vendor and caramel apples and cotton candy from the candy store across the street. It is the perfect seat."

"Last trip, we sat on the curb on Main Street for 1.5 hours waiting for the MSEP. We got great seats! Tip: Bring a sweatshirt, small blanket, towel, something ... to sit on. The cement ground is quiet hard!"

"Last December, while in the Magic Kingdom, we went to the Electric Light Parade. It was certainly a "don't miss' event! We had a perfect view and plenty of elbow room ..we sat just before and across from the Country Bear Jamboree. There was a path way to the restrooms in front of us(which is always handy with kids), and plenty of refreshments all around us! We felt like the parade was just for us! Every character approached us and the kids were thrilled!! (My brother and his family sat on Main street and struggled to see!)." Mary O'Neill

"I loved watching the fireworks at the MK from a table at Tony's Town Square Resturant. It had a great view and was extremly protected ( a good thing )!" David Miller