Yahoo is full of spam, hotmail is not used by anyone anymore.
Gmails invitation beta, instant messenger integration, gui coupled with the possibility of google wave integration and google apps for corporates and sme's has made it one of the most popular email service provider.
Ofcourse there are one's less heard of like inbox.com and paid ones like usa.net , I doubt if people are still paying for premium mailbox.
Freemium model is the way to go.