There are so many places to view IllumiNations at Epcot's World Showcase that are wonderful in their own way that its futile to try to list them, so I will.
Some great ones have been recommended in the earlier posts.
Those and a few more are listed below-
Bridge between UK and France
Near the twin gift shops at the entrance to WS from FW
Norway area
Upper level (outside front terrace) of the Matsu No Ma Lounge in Japan
Italy, near the Gondolas
Mexico, at the oudoor Cantina
Mexico, bridge to the right of oudoor Cantina (as you face the lagoon.)
UK, the Rose & Crown outdoor dining area (ressies required.)
UK, just to the left of the Rose & Crown (as you face the lagoon.)
Many, many more great spots.
Best Tip:
Don't stand down-wind of the fireworks.
Smoke will ruin the show.