I voted for Fantasmic for it's uniqueness to
Disneyland. (yes, I know it's in Florida, too, but ykwim!)
The fireworks are undoubtedly spectacular. Disney possibly does them better than anyone, though I understand some of your 4th of July can be quite spectacular as well. So, I don't necessarily need to come to DL to see fireworks per se....
World of Colour is, indeed, breathtaking. The scale it employs is unrivaled...but admittedly I saw the precursor to this show at the Bellagio in Vegas...again Disney does it much, much better, but it is not unique to DL....
Fantasmic is uniquely Disney. It is the ONLY place I can see a show like this....and the awe and wonderment it instills is unparalleled. I have to confess that we saw it for the first time in November, after numerous trips to both DL and WDW...and were stunned! This trip will be the first time our 21 year old son has seen Fantasmic, and I have booked the dessert package for our family. Can't wait to see his reaction!!!
All three are wonderful in their own rights...and are must do's every trip....but if I had to pick, it would be Fantasmic!