From The Mossberg Report:
As for how they rank, Yahoo Mail takes the lead. It’s fast, and its gigabyte of free storage is more than enough to free most users from deleting old mail. I also like Yahoo’s autocompletion of addresses, as well as its folder and filter systems. Plus, its overall user interface is clean and clear.
Google’s Gmail is also pretty good, though its quirky design could put off some users — it’s clearly still a work in progress. Gmail has the most free storage of the Web-based providers, which is a big plus, and searching all that mail is fast and accurate. But a simple operation such as deleting an e-mail takes more steps than in Yahoo. …
Hotmail comes in last. It offers only a fraction of the free storage of Yahoo and Gmail, which, for my money, flatly disqualifies it as a serious contender.