I think most things have been covered by others, just a few notes:
For standard browsing there is little to no difference between IE9, FF, Chrome or Opera. I use all of them to varying degrees depending on what I'm doing.
This means that for the most part, it's purely personal preference.
For me, I've been using Chrome mostly in the last couple of years, with FF and Opera installed for other uses.
However Chrome recently removed their 'experimental' side-tabs option and I personally hate using tabs across the top. It's a bloody stupid place to keep them with vertical screen space so sparse and horizontal in plenty (look at those lovely wide borders on the left and right, keep the tabs there instead
).
So I'm back on Firefox (which I used since it was v0.2 and called 'firebird' right up until I switched to Chrome) with the "Tree style tab" add-on that puts the tabs on the left in a nice format.
Opera I tend to keep with specific tabs open for coding or I use it when I have a game open because I don't want to open my 'main' browser as I have so many tabs open that Windows' terrible memory management causes the machine to become very sluggish.
What do they excel at? Well, Opera has a lot of additional tools, has side tabs and plugin blocking built in. Firefox has the biggest range of add-ons with the most important being noscript and flashblock (I sort of include adblock+ in here, but I'm not using it atm, but that's another story) which make the browser far more secure than any other when used properly. Chrome has the smoothest UI and a nice range of add-ons, blocks plugins natively as well. Firefox, Opera and IE all have significant delays when navigating around tabs etc, this is due to the way the programs work internally. Chrome's multi-process method means everything feels much smoother (although it's not rendering pages any quicker) but you pay a premium in resources for that.
So yeah, you pick the one that works best for you out of those 4.
Quick note on Safari: It's Chrome without the add-ons and with an Apple UI forced on it. If that's your thing then go for it. It isn't mine as I find the Apple UI hideous to look at, unintuitive to use and Chrome uses the same engine with a smoother UI and better add-on support.
Beyond normal browsing you can't beat Chrome though. It supports both WebGL and has its new 'native client' technology (Bastion in a web browser.
No, really).
Behind that you have Firefox which has WebGL support.
Then Opera which has just all round good support across the board with HTML5.
IE9 comes last in this case with minimal HTML5 support, but at least it mostly works with XHTML and HTML4 these days.
IE8... Don't make me laugh, it's not even in the same class.
As a final note for Chrome, if you're worried about google tracking everything, there is the open source version that has all the Google extras removed called Chromium. This is actually the build that Audiosurf uses (albeit through the awesomium framework).
