Christmas by far and away.
I don't know why, but something about Christmas in particular is like the EPITOME of joy and happiness to me.
To me, there's something so un-cynical about Christmas in comparison to the other Holidays (at least, I should hope so o_o)--sure, we've had it marketed as "Santa Clause's" holiday, but in all actuality, in my eyes it's all about the weather and the family and the food and the fun time together. And the religious meaning behind it, absolutely!
So call me a sap. I love Christmas like no other holiday! :>
Not to mention it's at the tail-end of the year, so it's so cozy and in it's little corner and happy to be the best of all celebrations. <3