Techcrunch has posted an article comparing GMail, the latest Yahoo Mail Beta interface and Windows Live Hotmail. All these applications are web 2.0 rich client applications and have huge storage.
Gmail is the newest player in this field but they are the guys who redefined email. Fast and responsive ajaxy interface, really good spam filters, labels (and filters to automatically label), autosave drafts, amazing search and of course, virtual unlimited storage. No wonder I converted to gmail for all my important email long long ago. Any important email on gmail, I am always assured that I would be able to it find it later. They only own around 1/10th of the market share, but they are still the best and recommended by techcrunch.
Yahoo is not my primary email account now, but I still use my yahoo id for its variety of services, in particular yahoo messenger. Yahoo launched its mail beta to catch up with gmail and it did reasonably well at that. They also opened their dot addresses which helped a lot to their huge userbase. However, their spam filter, which I once thought as the best available (before gmail i.e.), needs to improve. Particularly the false positives, which makes me check the spam folder for genuine messages once in a while.
Microsoft has been trying to catch up in this field as well, but looks like all their efforts have always been too little too late. I left hotmail long ago, and also left msn messenger when yahoo partnered with msn. To be fair, their new interface is pretty decent. and is along the lines of ms outlook. However, spam filters remains the biggest issue for it. Their spam filters look crude to say the least.
Overall, I would say GMail is a must, Yahoo is good but better in its other services and I would like to forget about hotmail as soon as possible.
Note to Rediffmail: How do u people manage to imitate all these new features offered by other providers and still be able to make them unusable?