I personally have never looked back since I started using Gmail back in 2004. Reason - well, it's simple, fast and logical. And not just now, but since I ever remember. I use it extensively with labels and filters which do make my life much easier.
I started off with Hotmail, but now only use it for Windows Live Messenger.
I started using a Yahoo! account as a "registration account", where all the spam could be diverted, but I must admit that Yahoo! has improved incredibly lately. Yahoo! would probably be my go-to-guy if I ever broke up with Gmail.
So, what do you guys use? Any tip you'd like to share which you think makes reading / writing email easier for you?
Oh, just in case I should add another option to the poll, please let me know.
Please note that this is not a thread to decide which service is the best, just to know which do you guys prefer.