If you haven't seen Wishes, I'd say definitely plan to see it. Then flip a coin between the other two, or go with whichever you liked better. Food for thought though, if you are time-limited for the trip, don't forget that for Fantasmic you have to line up early for the show, thereby losing park time (unless you buy the dinner package). For Illuminations, the "best" spots require getting there early, but you can almost always walk right up ten minutes before the show and get a decent spot.