I believe gmail is the best. It is light and have all the features needed. I have been using it for the past 4 years. I think it is the best one. Rest all of them take time to load.

gmail is simple , faster and most powerful .
they have good spam detection system .
yahoo is full ads .gmail contain only text ads and gmail login support many other google sites like
orkut anayltics adsense etc.....

I find it the most convenient and simplest mail service of all.
i have tried almost all the mail services and find gmail the coolest and the most user friendly of them all.
yahoo is good too but not comparable to gmail.

I don't use hotmail.Yahoo's mail is good but its loaded with flash and other ads.
I love the gmail interface. Its neat and clean. Professional too.

yahoo has been in the hearts of many people until gmail arrived. its all new interface, good mail search and tagging are good reasons to stick with it. you can create labels, and star the conversations and the biggest factor is the space. it surely impresses any new user. And the better part, it loads very fast. Even if your connection is pretty slow, gmail offers you a html only interface for that matter. If you dont have enough patience for it load, then you can try the rss feeds which would present you the unread mails in your inbox. And it does not bug you out with the old fashioned jeevan-sathi ads as there in yahoo which most users find annoying. So many reasons for choosing gmail.