I have accounts with all three (actually all four, i have an old hotmail account!) and have to say that they are each very good. The one I use the most is Gmail, but I have been very impressed with Outlook. I really like the free space Gmail and Outlook provide for storing documents and pictures.
I use yahoo mail mostly for spam or websites that require an email. it has worked well for me for that.
In my experience with all three, I get the least spam to my Gmail account. In fact, I get almost no spam to my Gmail account. But to be fair, I sign up for a lot of stuff through my Yahoo that causes it to get spammed. Outlook is looking good too though.
My suggestion? Use all three, they are free after all!