This thread again?
Anyway depends what I am doing.
Server. I do tend to lean towards Debian for this, unless it is something dedicated like FreeNAS,
General desktop use for me. Linux Mint for the most part but anything that supports XFCE I am probably good with, though mainly on top of debian. More generally it is like OSes in general -- if you have firefox, putty, gimp, lyx, libreoffice, scribus, inkscape, a programming notepad and a hex editor and something I can do some electrical and mechanical CAD in then I do not care as I will probably be consulting your documentation (or using the cheat sheet
http://xkcd.com/627/ ) to figure out how to do something on the back end anyway.
Usually mint or debian for clients that have linux on their machines (an increasing amount of them).
Security penetration. Obviously it has to be Kali but actually I have not needed such things lately so I am way behind.
Rescue CDs. Usually whatever comes with UBCD and/or Hiren's Boot CD.
Security for online banking and such like. If I absolutely have to then I will do something with SELinux support but for the most part a normal desktop one I lock down hard to do one job and one job only does for most purposes like this.
Lightweight for virtual machine. If puppy linux, something with XFCE or one of the above is not light enough then I do have a thing for openbox based distros.
Media player. Whatever XBMC/Kodi version works well for the installer I have this week.
More generally. I appreciate that some people enjoy arch and arch like things but life is too short for doing it as more than a learning exercise. I try to maintain a familiarity with everything on
http://distrowatch.com/dwres.php?resource=major plus puppy linux.