I haven't been to Universal during Christmas time so I can't really comment on that. But they were already putting up Christmas stuff in IOA in late October, so it's probably pretty good.
It's decent. IOA has ginchmas and Studios has Macy's Christmas Parade offered throughout the season. The decor is standard.
Sea World has
Very heavily decorated park, millions of lights at night and the decor does not follow the Disney ( giant tree, garland, wreath and a few smaller trees) standard.
Mostly* included with daily park admission.
Major shows:
O Wonderous Night
Winter Wonderland on Ice
Elmo's Christmas Wish
Sesame Street Christmas parade
Holiday reflections Fireworks
Additionally:
Rudolph's Christmas town.
Santa Claus meet and greet
Snow flurries
Holiday themed Streetmosphere acts
Sea of Trees
*Extra cost offerings:
Sesame Street holiday character breakfast
Dinner with Santa Claus.
Their own version of Epcot's Holiday kitchens all around the central lagoon.
All in one park.
I only stress it because there are so many people who really scratch their head at SeaWorld's Christmas. They can't wrap their head around that this is the park that actually does the most for Christmas in Central Florida by a country mile. There's just no way they can accept that. And then they go. And the reaction is always the same. Wow, That is a lot of Christmas.
I will say Disney has improved from a few years ago. The holiday menagerie at animal Kingdom Park is definitely a nice addition. The Magic kingdoms Main Street USA is always wonderful. I just wish there was more decor going beyond Main Street USA. Disney's Hollywood Studios has really up their game and I really do feel that is the best decorated park at Walt Disney World.
It is Epcot's holiday offering that I take the most issue with. Epcot feels so barren for the holidays. Even with the holiday kitchens the end result still feels like they decorated the front of the park, they put a Christmas tree at the World Showplace transition, and then a few booths around the lagoon. It feels barren and dark. A decor does not have the warmth and joy of the holiday season. It could be because the park is so big it's so hard to put enough decorations in it to make it feel good, but I'm not here to make excuses for them. Whatever they're doing it's not working. Epcot feels like a holiday void.