I'm with Gmail, always and forever. (maybe.) They are definitely the best, I love how they do things like

-automatically save everyone you've emailed with into your contacts list, deletable of course if desired
-saves an entire email conversation with 1 or more people into 1 file
-gives you customizable labels to mark each email with, I made mine things like "reply" or "urgent" if I don't have the time to read it.