Safari, Opera, Firefox, Chrome in order from most to least used.
Safari is my primary browser. Safari 5 is pretty nice with gestures and such... though I've always liked it on OS X, even on my old MDD running Leopard. (Wouldn't dare touch it on Windows, though.)
Opera is my "secondary" browser. I prefer to keep another browser open if I need another window instead of a second browser window - this allows me to switch to it with Cmd-Tab. I know of Cmd-`, but I personally feel as if I keep better track of things with a second browser. (This is also my primary browser on VMs, and my second browser of choice on Windows machines.)
Firefox I primarily use for testing things, and college-related items. (It uses a Gmail-based system, and I'd rather not have to log in and out of my primary email. It's also the "second browser window" thing I refer to, as well.) Aside from that, eh... I just find Safari more useful. (But should I be on a Windows machine, Firefox is always my first choice.)
Chrome gets use only when I need Flash - I do not have it installed otherwise, and do not plan to install it otherwise. I've only needed Flash a couple of times, thanks to MacTubes. (So glad that's Universal... PPC machines benefit from it hugely, and it lets me ignore Flash mostly.) Shame there's nothing like it for watching Livestream/Ustream/Justin.tv streams... and, of course, there's the occasional other Flash applet that there's no substitute for or Flash animation you want to watch. (On a Linux machine, I tend to pick Chrome/Chromium as my main browser. I used to use Midori, but it was just too crashy.)
I'll also use Elinks on a slow connection where graphics are best ignored. Of course, this situation really only ever crops up on the rare occasion I have to use a 56k connection, and in such a case I'm trapped on the old iBook. (For normal browsing on G4/G5 machines, now, I use TenFourFox and Camino.)
...Boy, I really overthought this, didn't I?