I think Fantasmic. I love World of Colour so much, but it is never a problem to get a good spot to stand. And you can sit on the ground while you wait, which isn't the same as sitting the whole time, but does help.
Plus I am just not enough of a sweets person to justify it. Maybe if it was for like a girls' trip or special occasion? We live out of the country and just have never been able to justify it on any of our trips, but since you said your trip is likely a one-and-done, maybe that is worth it?
Otherwise, Fantasmic is absolutely the way to go, and I agree that Rancho is solid value - that's good food, and a decent amount, and a good view for the show. Plus, as others have said, stay there and watch the fireworks! What an awesome evening!
You DO have to sit on the ground... I'm not overly precious or concerned about that, but of course, it isn't as nice as sitting in a chair, so that just comes down to personal preference. I have no desire to fight the chaos of Fantasmic open seating after walking all day, and with our heights (I'm very short, DH is very tall), jostling through all that is always kind of awkward. Much rather just grab some beignets and a julep and sit on the ground and watch the show.

We've done Blue Bayou as well, which was great but obviously much more of a splurge. And way back when, Hungry Bear had a package too that was worth it. But nowadays, I'll do Rancho every trip if I was so inclined.
You just have to prioritize which show you want to watch, and how much sitting in a chair vs sitting on the ground matters to you personally.
Either way is a win!