Definitely Opera. You have to learn a new browser interface though, and a lot of people are happy with Firefox so not many transfer over.
It's also a matter of preference. Opera is a lot smoother, sleeker, and faster than Firefox, but Firefox can be tricked out with all the add-ons if that's your thing (Opera can too, but there aren't as many since less people use it). Personally I like my stuff fast, convenient, and easy... Opera fits that bill perfectly. Firefox has too much bloat for my taste.
Having used Mozilla Firefox, Internet Explorer, and Opera I definitely recommend Opera... then use Internet Explorer when needed. The new IE actually isn't that bad in all fairness.
I like Mozilla's email client Thunderbird, however.