I have to say I like Gmail better.
It seems to run more smoothly for me than Yahoo and its also easier to keep your emails organized.
I also get much less spam in Gmail, plus Yahoo has those annoying ads that take up a quarter of the screen and every time I close the ad a box pops up advertising "Yahoo Mail Plus" then it says "click here if you don't want to see this again" so I click there but the next time I close an ad, there's that box again, how annoying
I don't use MSN for emails but I'd have to say it beats Yahoo and Gmail as a messenger system.