Firefox on Windows, Chrome on Linux, Dolphin on Android, Mercury on iOS.
I would like to use Chrome on every platform, and I feel like it will completely take over the browser market in the next few years. But I get extremely annoyed by it's quirks and it's relatively poor extensions (Firefox's ieTab implementation is much better, and chrome doesn't have a FlashGot equivalent. Not to mention that Xmarks routinely ****s up bookmark sync on Chrome). I run Linux on my old, slow computers; Firefox is noticeably slower, so I use Chrome there. But on my fast Windows 7 machine, where I do most of my web browsing, Firefox reigns supreme.
Dolphin on Android just kills. Nothing else even approaches it.
Mercury has nicer gesture support and more options than other browsers on iOS. In-built adblock is a very nice perk.