Visio is my favorite, but I'd have to say Excel. OneNote, Word, and Infopath come in after that. Publisher and Powerpoint are only used on a few occasions, and I try to stay away from Frontpage. I have never used Access.
I'd have to say Word is the most common one I use, since I have to type a lot of papers, and that's what I use. I occasionally use Publisher and Powerpoint as well.