Cwm as my backup WM (I believe it is the smallest WM out there, very simple too). I usually work with spectrwm, which just feels and works like i3-gapps but… With more quality(?) It just feels better.
I soon may also use IceWM. I like the old feel of it, anyone can use it if I hand them my computer and (finally) the developed version of IceWM has been put in the repos