Gmail is convenient for me just because it has a nice android app. I don't really like yahoo much, it works but a lot of my email needs are handled via mobile and their android apps are absolute garbage.
I used yahoo for a few of my accounts no regrets there. I recommend staying away from hotmail it seems to lock my emails quite often due to emails being sent which I don't respond to. Gmail also works really well and I use it for my everyday uses.
I had a pretty unique @oath address from www.mail.com which is pretty cool. www.hushmail.com is the most secure and best for signing up to things you could do without being hacked like Sythe.
Yahoo & GMail are the best functionality wise.
Outlook/Hotmail is alright but I find it awkward and slow. Not to mention it's not very secure at the best of times.