I used to be a big Yahoo mail fan, they served me well for years. But somewhere in all their constant UI updated to it over the years, I felt it got clunkier and slower. Somewhere in one of these transitions, they lost the first five or six years worth of all my saved emails, as well. I don't know if they've improved, but I definitely wouldn't personally go back to them.
AOL is simple enough and works fairly well, or at least that's the vibe I get from the two or three brief times a year I have to check mine. 😛 I've never used Outlook though.
I have a gmail, but it's my spambox. I always hear people rave about the supposedly great spam filtering gmail has, but it has never worked for me worth a darn.
Nowadays my primary email is a small, free IMAP provider through Mozilla Thunderbird. Best email account I've ever had. 😀