Man, that is like choosing who is your favorite child.
If you have 3 nights, why not see them both? The Disney nighttime shows are spectacular. It's not just pyrotechnics, it's the story and music and how it all comes together. Anyone who is not moved by them is someone I don't want to hang out with!
If I REALLY did have to choose just one, for me, personally, my choice would be Illuminations, but I'd bet I am in the minority. I love the music, how it ebbs and flows and tells such an epic story. I also like how it's relatively easy to find a good spot not too far in advance of the show.
But Wishes is in many ways the essence of a Disney vacation. When that first single firework arcs across the sky, it evokes a feeling like none other. Although there are many good viewing spots for this show, kids on shoulders can be a huge problem for your view if you are stuck behind one.
Enjoy your trip!