Hotmail - first one I had, pretty good even with M$ ownership;
Excite - also pretty good; lacks the "done" button;
mail.com - got it as an NBCi member; not too bad but not as fluid;
Netscape.net - AWFUL. I dont recall using this much because 99% of the time it's simply unavailable;
thepretender.com - so you know whatta fan I am
planetout.com - incapable of handling pictures. enough said.
I use a Mac, so I also get mac.com email that operates only through email programs on the computer (correct me if i'm wrong!), and it's been great.