Bitwig because it looked pretty fun to work with, and I like some features I miss in Studio One.
Renoise because I miss Fasttracker II. It loads the .xm format, but won't save in this format, even though there is an addon that can export in the .xm format, but it doesn't work very well with the songs I have tested it with.
Reaper because I needed something that wasn't as CPU hungry as Studio One can be. There is a huge difference using certain plugins.
FL Studio during the Call to Arms, I'm a bit curious of what the fuzz is all about. And there are a lot of tutorials on YouTube using FL Studio.
Reason because I just like how unique it is comparing to other DAWs. It was also the DAW I used for my track ever (not counting Fasttracker).
But, Studio One is still Studio One. Even though it's a CPU hog from time to time, nothing (and I mean NOTHING) beats its workflow. So I could probably never replace Studio One as my primary DAW.