Opera has the best functionality out-of-box and it is more standard-compliant than FF, but Firefox can be made to work in really similar fashion with pretty little effort...and there are other browsers that pass the Acid2 test (like Konqueror and Safari) and neither are in this poll so the importance of passing Acid2 test is questionable in this context - it does not make the whole browser automatically better than those that don't pass, eventhough standard-compliance is a definite bonus.
Also. Opera has a history of getting into 100% CPU load with javascripted pages, even when said scripts should be doing nothing, so that's kind of a minus... same with some menus (like Hard Light's), there are some gliches in how Opera renders them.
So I'd say that Opera has the edge on immediate usability, layout, efficiency and downloading system, while Firefox has significantly better customizability, is open source, efficiency difference is so small that on modern PC's with CPU to spare it hardly matters... And there's a portable version of Firefox, which is dang awesome to have with you.
I used Opera for the longest time, but it just kept putting the proc to 100% on the pages I frequented to, so I swapped to Firefox, spent less than an hour getting it customized (All-In-One Gestures, AdBlockPlus, Free Download Manager plugin and Gradient Brushed Metal theme)... and now it works remarkably same to Opera, using the same mouse gestures.
So... I'd say there's no such thing as best browser, but IE is certainly one of the worst, following truely the software engineering principles behind Vista.