The "official" speed test for browsers tends to be based on running the ACID 3 test.
ACID 3 tests javascript compatibility and performance as this is considered important these days as all these new online apps use javascript as their main language.
The first browser to score a perfect 100/100 score on compatibility was Safari 4.
Last I read (which was a few months ago to be fair) Safari 4 was also the fastest:
http://www.betanews.com/article/Apples-Safari-4-Beta-for-Windows-speeds-up-after-security-update/1242321496Bear in mind these tests are all done on PC's, and you may find some difference between Mac and PC versions of apps (eg I find the Mac version of iTunes runs much better, whereas I find the Windows version of Excel to be more stable).
Regardless, be it Opera, Chrome, Firefox or Safari, just use whichever you prefer. Just don't use IE as it's a security disaster and doesn't render web pages to standard. Not even close.