For me, IllumiNations hands down.
I loved the castle projection show before Wishes but the fireworks were just okay.
We didn't see Fantasmic so I can't compare it.
If you judge it based solely on the fireworks themselves, I would say Wishes at MK is best. But if you judge the entire package (fireworks, music, atmosphere) then Illuminations at Epcot is best. I am a huge fan of Illuminations, based on the entire presentation.
And just note that the fireworks at a special event like MNSSHP or MVMCP will have an extra flair, but I see OP is not going at that time of year anyway.
nothing beats illuminations in my opinion.
you are so close to those fireworks that it vibrates right through you which is one thing i look for in a fireworks display.
also i love when the fireworks light up the sky like its daytime which happens at the finale.
the the song "we go on" is just amazing!
i tear up every time.