At this very moment, I'd say galeon is best (though I've had trouble getting it working in the past; still, after it is up and running it's a good browser. requires mozilla though).
At the rate it's progressing though, in another 3 weeks Phoenix will be best on Linux and give IE a good run for it's money on Windows (you'll be able to use it without looking back if you're trying to go open source on windows. I know that's a contradiction but some people want to use open source they just can't bring themselves to dump windows). It's also a lightweight mozilla-based browser, but it seems to hack around with the mozilla code more. Galeon requires mozilla be installed, whereas Phoenix just includes whatever files from mozilla it uses. Give it a try. Version 0.2 is currently out and version 0.3 is supposed to come out tommorow (nothing guaranteed on volunteer projects though). I can almost guarantee I'll be using it exclusively on Linux very soon. The same team also mentioned creating a seperate, slim version of the Mozilla mail client as well, which would be an awesome thing. Kmail is good as it is so I won't complain (Evolution is a little too bloated for me. It's a great Outlook clone but I've never liked Outlook either), but choices are always good.