As an adult, Christmas and Thanksgiving are the only two holidays that mean anything to me. The only two holidays with something extra associated, culturally or in terms of family time; we never did big family events on the 4th or Memorial Day. I haven't been awake past midnight the last couple NYE's. Halloween loses it's appeal once you're too old for the "let's go to a party where every girl has on a slutty costume" phase.
Christmas is tops by a mile. My entire family is a bunch of ex-Catholics but I still love the pomp and circumstance of the whole ordeal. The holiday is even more enjoyable when 8 AM Mass doesn't make your hangover 10x worse. Even non-religious people are generally in good spirits that time of year. The tree/lights, opening gifts, the music (most people hate it, I love it), the food (better than Thanksgiving in our house), the old movies that run are on repeat on TV all week, seeing family. It's the most festive time of year and I love everything about it. My job gets very quiet the week between Christmas and New Year's which is an added bonus.