Mozilla because its the fastest for all the things it has built in... however I'm trying out Safari right now and it seems to be working well too... juts the stupid no center click, and doesn't have all the add ons that you can get in mozilla which somewhat kills it for me. EDIT: and apparently there is a security breach in safari so thats gone now xD
Opera, Damn fast online and computer based, mouse gestures, nice little layout and has some really sweet themes and widgets too
Opera ftw, I tried to use firefox and I just hated it. Way to slow, and I just love the feature of goring back pages with mouse 1 and 2. Plus it comes with a bit torrent client and mail app.