All three are completely different experiences.
Wishes is a “traditional” fireworks show with added special lighting effects on the castle. Illuminations is a combination of fountains, lights, lasers, fireworks, and floats. Fantasmic is an outdoor stage show featuring Mickey and a host of other Disney characters with lots of special lighting, water, and pyrotechnic effects.
I haven’t seen Wishes from the Poly, but the view was good from Narcoossee’s (not as much so from the GF marina). Another option is to watch from the “public” viewing deck at the CR. The “best” view of Wishes, is IN the park, along MS. The show is “designed” to be viewed from that angle. That being said, MS during Wishes is a nightmare crowd, you have to stake out (and defend) a spot at least an hour in advance, and still have no guarantee that someone won’t hoist a kid up on their shoulders just as the show is starting (speaking from experience). On my last trip I watched from the second floor of the train station and it was a very good view, it also makes getting out of the park and to the transportation much easier, as you are at the front of the crowd. The FPP location is a very good trade-off between viewing angle and crowds. It’s off-center so you don’t get the “picture-perfect” view, but you can show up 15 min before show time and watch in a relatively uncrowded location (that even has grass to sit on) and you get a very nice view. You’ll still have to fight the crowds exiting the park at the end of the show.
You can only see Illumination if you are standing around the WS lagoon. You might glimpse a few of the higher fireworks for the YC, but to really see the show (which is mostly low) you have to be right by the lagoon. You’ll need to find a spot 30-40 min prior to show time. MUCH less insane than Wishes.
In either of the above, exiting the park after the show is is crazy with crowds; it will entail a lot of patience and hand holding to not get separated.
Fantasmic is a show with a central stage and ampitheater seating. It has some fireworks at the end, but it’s mostly about the characters performing coupled with special water, lighing, and pyrotechnic effects. If there are 2 showings on the night you are there, the second will be MUCH less crowded. If there is only one, you will need to line up 45-60 min advance to get a good seat. If you are late, you might get standing room only, or may not even be able to get in.
I never miss Illuminations; I love the show, and it is “relatively” hassle-free to watch (and I hang out in WS after to avoid the exit rush). The other two are hit-or-miss for me. Wishes is a beautiful show, but the masses of rude people on MS have kind of soured me to the whole experience; in the future it will probably be FPP or nothing for me. I’ll see Fantasmic if I do a dining package and can enter 15-20 minutes prior, but I’ve seen it enough times that I am not willing to spend the time it takes to wait for the show.
These are just my personal observations. I hope they help in your decision making…