I have recently read a lot of reviews/post comparing the three email giants with all the upgrades these three went through in the past months. I do have an account on all of them but i primarily use YahooMail and Hotmail. Anyway, here’s my user point of view on the three:
:: speed
gmail = 3
yahoomail = 1
hotmail = 2
I usually experience YahooMail being the one to load faster. Gmail and Hotmail always take time to load. This is based on my experience at home and at work.
:: ads
gmail = 1
yahoomail = 2
hotmail = 3
Opening my mailbox on my browser, i could say that Gmail has barely any intrusive ads. I hate YahooMail ads on the frontpage and the one that comes in the signature while in Hotmail, i hate the huge banner add ontop of the page and the one on the frontpage too.
:: user friendliness
gmail = 3
yahoomail = 1
hotmail = 2
On matters of organizing my emails and other content, i find it easiest to do in YahooMail, very fast and hassle-free. Hotmail and Gmail comes next.
:: layout/look
gmail = 3
yahoomail = 2
hotmail = 1
Taking a look at the present layout/look and how you can manage it, i like Hotmail best. The look is so nice, elegant and clean and its very manageable. YahooMail looks fine while i think Gmail needs some improvement coz it looks dull for me.
:: security
gmail = 3
yahoomail = 2
hotmail = 1
When it comes to spam protection, i observed that Hotmail works best based on my own experience. I barely get a spam/junk mail on my inbox with Hotmail though sometimes i can’t figure out why some emails coming from my safe list sender are still goingto my junk folder. In Gmail and Yahoomail, i still get spam/junk mail on my inbox daily and most of the time they are the same spam/junk mail i get from time to time.
:: storage space
gmail = 2
yahoomail = 1
hotmail = 3
Currently, Gmail offers 6GB+ of space while Hotmail offers 5GB and then YahooMail offers a virtually unlimited amount of space. Hmmm.. but anyway it doesn’t matter much to me since i’m used to emptying my trash often coz i like keeping my mailbox clean 
In summary…
Gmail – I barely use my gmail account coz i find the layout dull and boring plus there isnt much to do inside the mailbox and i dont feel comfortable using it.
YahooMail – This the one i use primarily. I love YahooMessenger thats integrated within the mailbox. I feel comfortable browsing around since everything is easy and fast to use and the layout is fine.
Hotmail – I love its elegant, clean look and easy way to change the layout color. What i love about Hotmail is the multiple email account integration that allows me to open my other hotmail account without having to logout and login from one account to another. Its pretty helpful and convenient.