In my opinion, Wishes at the Magic Kingdom is not to be missed, and is best seen on Main Street the first night of your trip. Why? Because is is such a feel-good presentation that infuses you with Disney magic and is a perfect end of a perfect first day at the theme parks. (If you are able to spend a week at WDW, I'd also see it on other nights from other spots, such as the Polynesian beach and from the Contemporary resort, but only after it's been seen from Main Street.)
My second choice would be Fantasmic, because it is a unique show. There is stadium seating and is more of a show than just fireworks set to music. The characters are there to present a story, which also has fireworks and lasers.
I know many people are fond of Illuminations, but it is my least favorite show, perhaps because I remember the show of long ago and the current edition is a pale shadow of its former glory. It's a moving experience as well and if you had 3 nights I would recommend seeing all thee. But if one had to go, this would be the show.