Well, you want to go full throttle nerd rage on me - you got it:
Safari - the only "so called modern" web browser to lack :
Hardware acceleration
Plugin support,including adblocker (
http://www.apple.com/safari/download/plugins.html - doesn't count)
Bookmarks sync - Firefox and Chrome already doing it
But the bottom line ,I don't have Mac pc and maybe Safari 5 is somewhat decent on it, but I'll stick on using windows and lets be honest ALL apple software for windows are just POS.
On Windows box, Chrome imo is still the best all around browser, closely followed by FF4. IE9 made huge leap with the newest version, so if you love IE for any reason and have vista/7 - you must update to ie9 asap