This is a tough question and its hard not to be biased because I have accounts with all three of them, and each of them have there very own benifits.
First off Gmail is by invitation only, it provides a better user interface, more file storage, and larger attachment files then Hotmail or Yahoo does. In this manner if you are looking for a proffessional and powerful email service then Gmail is the way to go.
Second is Hotmail, hotmail is free but there user interface is not that friendly, they have kept the same old look since the beginning, you cannot send very large file attachments, I find that you get much more spam with hotmail, however if you use MSN messenger for personal use then hotmail should be your choice of a email service because you dont need to use your email address for anything else, keep it as a dummy address, empty out the spam every once and awhile, and use it to talk to people you dont know personally this way you dont wreck a good email address.
And last but not least is Yahoo, well I put Yahoo in the middle of the two (Gmail Being The Best), Yahoo does have a friendly user interface, and they allow for a moderate file attachment size, there POP access is super easy to set up, and they offer more things then plain old hotmail. Yahoo is an excellent personaly account for anyone who does not usually do a lot of things online.
So it really depends on what you need the E-Mail account for. If you need it for MSN and some chat sites then go with a dummy Hotmail account to prevent spamming your good accounts. If you need it for a basic everyday personal use where you wont be requiring a large amount of traffic then go with Yahoo. But by far if you need your email to handle the traffic, offer a good user interface, get the proffessional look, and be able to send larger files (The more interactive online user) then Gmail is the email of choice for you.
