My favorite sensation is the lack of one. I like when I'm so engrossed in something, so lost inside of it that I almost cease to exist. This happens when I play music, at least when I'm doing it right. I suppose the sensation is complete peace. It's not happy or sad, it just is.
I think this is because that during such times, the self-analytical inner monologue dies away. You're not thinking 'this is amazing' or 'what fun I'm having'. Like you say, it just is. And you only realise how profound such an experience is once you're analysis returns and you reflect upon it. It all goes by fast. It's a blur. On trying to recall, you perhaps can't even picture what exactly transpired in such moments. All that remains is the desire to return to that state.
I spent Time the other night just making noise on the strings of a piano, getting sounds of it I never had before. I was definitely not thinking, but then as I walked from the piano I said aloud "that was awesome." I didn't realize I had said it until later when I was listening to it and mixing. It was cool to hear say that because it was like I had "returned" and realized what I had just done.