There was a time when I would have said Fantasmic! hands down, but I did not care for the new version, and would now say the WDW version is better. I would not miss Paint the Night, and I would try to do Together Forever, which is best from the Main Street area. It is definitely possible to do all three in two nights though, either by watching TF from the F! area, or doing the later show of F! after the fireworks. It is also kinda possible to make it from PTN to TF in time to watch from the back of Main Street, but it's a gamble (and you need Hoppers for that). That's a race for sure!
One thing I do recommend is that you try to do TF on your first day because it could be cancelled due to wind. Then if you want, you could try again on the next night. Anyway, all the shows are great so you can't really go wrong.