IE when I have to, but only then. It has been many versions since it actually responded to user input in a timely fashion. I'd be willing to use it more, if they'd actually make it respond. Why they don't is beyond me. FI, last time I needed to use it, I opened a new window, hit escape (stop), got focus in the address bar, typed in the URL I wanted, hit enter, and...oh, wait, it's still grabbing stuff from this current page load, and is going to keep doing that, then set the address bar contents to the current URL, and ignore that you, the user, actually did something. Grrr.
Non-Chrome Chromes, lately, due to extensions finally getting there. However, font rendering on Windows leaves something to be desired, and there doesn't seem to be a Youtube DL extension, even 3rd-party, that works. Also kind of annoying that their own distribution blocks such extensions.
FF still does the job, and I like the new PDF reader, but I never know when I'll get a connection reset, or it freezing for no good reason. The HW accel is also jerky, and can induce audio glitches, in Windows 7 (not directly a FF bug, but it's an issue, nonetheless). Also, they're still working on fsync() bugs, causing some sites be slow/jerky, instead of scrapping their DB implementation and doing it better (which would probably take less effort, by the time they're done).