M, most of the time.
In the past 4 weeks I've probably shot on everything including Full Auto.. I was taking pictures of my car interior and just needed some quick snapshots in bad lighting in my garage... Green Rectangle saved the day of pulling out flashes etc.
I'm always hesitant to switch modes, because I have a hard time remembering all the dials and their different function in the different modes.. but there are some times that necessitate it..
Use what you're comfortable with and what allows you to take great images. There is no best mode for every person. (This bears repeating)