From my point of view Opera is the best. It's fast,smooth renders well and has a download manager,feed reader,email client and torrent downloader all inbuilt !
Google chrome and chromium are pretty good too and are fast,snappy and smooth.
However, in my computers I always found that flash worked best on firefox i.e firefox was able to play flash video smoothly and didn't get slowed down even if many flash advertisements were present (ok so i blocked them using noscript and adblock).
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