Most of our mission critical Macs are on Mavericks due to continued stability issues with Yosemite that I still see. It's gotten better but the bad feeling is still there. Hopefully El Capitan fixes that for me. Most of the issues deal with WiFi/Bluetooth/Networking bugs rather than actual crashes though. So if El Cap refines that I'll be cool with it.
My personal favorite is still Snow Leopard though. That probably won't ever change since I spent the most time on it by far. My families iMac shipped with Tiger but Leopard came out later that year so I spent 2007-2012 using Leopard/SL. Ill be upgrading to 10.11 when it releases and it's currently on Mavericks since I went back from Yosemite due to annoying frame drops and blurry text issues. But still, Tiger to El Capitan on the same machine is impressive for Apple's standards.
I also like Tiger since it's still quick on my old iBook and I kind of miss the brushed metal/aqua look, but that's not really relevant now.
Edit: I forgot to say what Macs I'm (primarily) using.
Personal: Late 2013 13" rMBP - Yosemite as of last month, was on Mavericks
Work: Early 2015 13" MBA - Shipped with Yosemite
Home desk: Mid 2007 20" iMac - Mavericks as of yesterday, was on Yosemite
And lots more I maintain at work (42 Macs 2009-2015) or for family members/friends.
For family: Mid 2007 iMac, Early 2008 iMac, Late 2008 MacBook, Early 2009 MacBook, Mid 2009 13" MBP, Mid 2010 15" MBP, 2x Mid 2012 13" MBP.
ALL SSD equipped because of me. Haha.