Opera hands down even though i'm using firefox right now. I just can't get away from all the Kah-raze-ee plug ins that kick so much ass. Firefox is a resource hog big time though. Opera is hands down the fastest. I just tested this theory out on my girl's mom's OLD ass computer, it's a 4 year old toshiba satellite and opera runs SMOOTH on it. For all the bad stuff out there on the net nothing compares to good old common sense, a good antivirus (NOT Norton or MCAFEE), peerblock, and a good firewall (built into your router).
Keep in mind the "footprint" of a browser is what makes it such an attraction for malware. When the market is flooded with IE and Firefox thats where you'll see your viruses hit. So the next time an apple fanboy (i love mac BTW..just not a fanboy) runs his or in my case HER mouth about macs don't get viruses tell them that when they get the market share good thing await them.