We love all three and try not to miss any of 'em while at the World, but if I had to pick just one.....it would be Illuminations, no trip to WDW would seem complete without it.
I'm picking this one because the fireworks, lights, lasers, globe picture screen, water works and music are unbelievable. Plus, you don't have to spend alot of time waiting around on Illuminations. In fact, if you set a priority seating at the Rose & Crown for around 8:00 p.m., there's a good chance that you can sit outside and enjoy a nice meal with desert arriving just in time for Illuminations, allowing you to sit comfortably and enjoy the show without any interuptions.
Catching Spectro or Fantasmic usually involves an hour long wait trying to save a good spot/seat for the show where other guests will probably crowd or obstruct your view. They do offer dinner packages for Fantasmic, but the seats aren't all that great.
That said, I can't stress enough how increadible each of these shows is. Each is truthfully not to be missed. Fantasmic is unlike anything you've ever seen; mixing fireworks and lasers with film projected on water screens and an enormous live action stage, the largest dragon I've ever seen, more disney characters than you can throw a stick at and of course, Mickey as the hero of it all.
Spectromagic is one of the best parades WDW has ever done ( I still love MSEP ), and if you catch it, you can probably catch Wishes too which should never be missed. In fact, catching these two shows in one night would be hard to beat.
As you can tell I'm a little conflcted about all this, but I'll still stay with Illuminations for the total experience. Good luck, and I really hope you're able to do them all.
