Professionally I use Outlook, as I was advised to by the company I work for. Personal usage goes to GMail, which I've used for about ten years on and off. It seems to be the most reliable, and the Android app helps me keep on top of things when I'm on my phone.
I use Gmail for important things, such as my Amazon account, bank account, credit cards, etc. I use Outlook for things such as joining forums or other types of websites -- anything that I don't want to be bothered by and have clog up my important email accounts.