Well, mostly I say "Merry Christmas", as you can see by my signature line, lol. I normally use them all in some form or other I think though.
I do use "X-Mas" from time to time, although whenever I use it I feel a tad bad about it afterward because there was also a point brought up years ago that when the term "X-Mas" was used instead of the word "Christmas" it was almost like trying to take the word "Christ" out of the Christmas holiday.
I have never had that intention and most certainly would never think of that. My reason for using the term "X-Mas" is that it was a shorter word and therefore quicker to write. So, I'm lazy sometimes, but when you write a lot of Christmas Cards, etc., sometimes you look for a quicker way to wish people Christmas Cheer, ya know? Me bad, I guess. I really had not thought of it any other way. I also think that the "nincompoops" (pardon my whatever) who come up with this termonology or reasoning about why you can, or can't, use whatever word or whatever it happens to be, really need to get a life and find something worthwhile to do with their time and stop harassing everyone else.
I really don't think it matters how you spell, write, or say "Merry Christmas", it is the thought in your heart that matters most.
Oh yes, sometimes I do use "Happy Holidays" or "Season's Greetings" if I want to wish someone a "Merry Christmas" and a "Happy New Year" together. I mostly just stick with one holiday at a time, though.