Firefox primarily - noscript, adblockplus, ghostery, tree-style-tabs are hard to live without (although I think chrome now has integration/variants for most of these)
Chrome as a backup for websites which don't play nicely with firefox (mostly poorly written forms for insurance etc.)
I use linux primarily so IE wouldn't be an option for me, although honestly I still wouldn't use it if I could - prior versions because of security, but that's somewhat leveled out now. The new version I don't like the UI and again, no plug-in support.