For "Oh Holy Night", I was going to say the best "recent" versions have been done by Celine Dion or Clay Aiken...but my goodness, you guys are right, you can't go wrong with the classic Nat King Cole version.
"Baby It's Cold Outside" is a good choice to discuss next, as there is indeed multiple versions of that! I don't neccesarily have to be the one to toss out the song choice, but I do want to make sure each song is given a few days to be discussed (I love to hear everyone's opinions).
So for "Baby It's Cold Outside...", there's a version by Rod Stewart from just a few years back that I'm actually pretty fond of (and I usually can't stand Rod Stewart!)
It must be his duet partner, Dolly Parton, that makes it work for you.
I kinda like Ringo Starr's version... he even makes a mistake in the lyrics and corrects himself and they left it in the final cut... reminds me of caroling when at least someone couldn't remember the lyrics and messed up everyone else... good times...