Without question, Christmas.
Halloween is cute, but very much designed for the littles: whimsical, childlike fun. Trick-or-treating, colorful characters, under-the-sea bubbles, free candy. Most of the festivities occur during the day, and its still very hot around that time of year. The sunshine and crowds and humidity can be oppressive when all put together.
Christmas is pure magic. Oh Wondrous Night is NOT TO BE MISSED, simply one of the best holiday performances I have ever seen. The park takes on a completely different feel after dark, and the weather is perfect. The Christmas shows are all beautifully done and completely different from the daytime shows and holiday music fills the air throughout the park. The Rudolph characters are adorable and so much fun, and you can meet Santa and get photos taken. To top it all off, it snows several times a night in the Waterfront. And you can get your photo taken inside a full size snow globe!
Christmas at SeaWorld is THE BEST!