It's Google for me, don't know if I'm used to or what, but I can never find quality results on the other SE. Other than that, I find GMail and Google Analytics extremely useful.
I always seem to prefer Google. Since I use them as my homepage (their personalized page actually) I usually use Google. Although many say that Yahoo's search capabilities are as good or in some cases better than Google, I don't tend to use yahoo all that much.
