I'll avoid discussing what I use at work and on my personal servers, and go over what I have used as my primary OS on my primary desktop/notebook/PC.
1) Redhat - 1997ish - Wasn't my style, so it didn't last long.
2) SuSE - 1998-2001 - Nicely polished; I only stopped using because of new hardware.
3) - dark ages - 2001-2002 - I toyed with lots of distros, but none of them really did what I needed for my hardware. Windows was primary during this timeframe.
4) Gentoo - 2002-present* - Gentoo has been my go-to since '02. There was a period between 2006-2007 where it was seemingly in "meltdown," but those days are past.
*5) KUbuntu - 2006-2007 - I needed a working OS, and Gentoo was it during this timeframe.
*6) Fedora - 2011 - I tested Fedora on my desktop last year for a few months before I abandoned land-locked PCs and went full time to a laptop/netbook.
*7) Arch - 2011 - I also tested Arch on my netbook, but it didn't like the poor little thing for a multitude of reasons.
*Also, as of June of 2012, my primary home PC is a Mac.... So my primary home OS is not, and probably will never again be, a Linux distro. But this isn't the place to discuss that....