Christmas takes it for me. The music, the gifts, the snow, the decorations. Just amazing. Halloween comes in a close second cause I love all the candy & costumes. And then Thanksgiving.
I have to say it's an equal three way between Halloween, Thanksgiving and Christmas because it's all in the fall/winter which is my favorite time of year.
Halloween actually probably gets better as an adult. Trick-or-treating is just a very small portion of the holiday. Go to adult parties and you can get just as much candy and free alcohol as well. You get to be way more creative with your costumes and get to do a lot more because you're an adult and a lot of stuff available for the holiday is for "mature audiences".