Compiling a ranking based on my experience with all of them:

1. Yahoo Mail — best spam filtering, 6MB storage, fast interface
2. Hotmail — huge user base, 2MB storage (pathetic), spam is bad
3. Excite Mail — decent but who uses Excite?
4. Lycos Mail — meh
5. Mail.com — nice domain options but slow

What would you change?

Yahoo Mail is the clear winner right now. 6MB is generous and the interface is clean. Hotmail has been going downhill since Microsoft bought it.

I'd put Mail.com higher — having an email like yourname@programmer.net or yourname@engineer.com is pretty cool.