I wish I had an uncomplicated answer for this.
My favourite holiday has long been Halloween, from before becoming a Witch, because it was the one holiday that people couldn’t ruin for me. I hated Christmas with a passion.
Now that I’m older, I find I don’t actually celebrate Halloween in a pagan fashion. The last ritual I went to for Samhain was horrible, ending with me having a migraine and some serious issues with the temple. This year I may not celebrate at all (as much because I’m moving at the end of October as any other reason).
I’m starting to like Yule more as time goes on. Beltane is also a favourite.
But I think the most important one to me, and the one I want to answer this question with, is Imbolc. It’s the day dedicated to my Lady Brighid, the day I dedicated myself to her. The day Her light heals my broken soul, reforging me into something useful.
It’s quickly become the most important holiday of the year for me, and one I celebrate without fail. The others, not so much.