I use Microsoft Virtual Earth 90% of the time against Google Earth, mainly for the great Birdseye views :) Yahoo maps doesn't really do it for me, but occasionally I plug it into TileProxy for FSX (for flight sim purposes) as it gives good coverage of the US.
Google search is used 90% of the time against Live Search. Not that I would say there is much difference between them, its just my fingers are in a habit of typing 'google' for searching. Yahoo isn't used here at all.
I use Yahoo Mail the most, with my secondary account being hotmail. I have tried GMail, and can't honestly see why everyone raves about it. Yahoo Mail blocks out most spam, and provides a great interface to use as well :)
So, in conclusion, I use each company the most for a different purpose. If I had to state one company that takes the prize, I would probably say Microsoft, as I use its Video and Academic searches quite a lot too.