It varies with the shot. 3:2 (or 2:3) has a good balance - it is quite close to the "golden ratio", long established as a harmonious set of proportions. If I'm producing desktop wallpaper or a background, I'll have 4:3 in mind as that is still the ratio used by the output systems I am used to. Sometimes I will feel that 16:9 works, for a cinematic feel, or 1:1 (square).
To be honest, I rarely stray outside those proportions (I also normally try to crop to a neat pair of dimensions, like 2400x1800... that probably says a lot about me!
). It frustrates me that my D40 outputs images at 3008x2000 - if I decide to edit the image, I will invariably trim off the extra 8 pixels even if the overall framing is fine!
Wulf