I've been using Cubase LE AI,it came with my interface.I like it so far,it hasn't given me any problems and,after watching a few videos,I don't have any trouble working with it.It's got all of the basic functions I need and it lets me use all of my 3rd part plugins.I was disappointed when I tried Studio One Prime and it wouldn't let me use my plugins.
I just downloaded the demo of Studio One 3 Pro and will be trying that out for a few days.I tried Reaper and just wan't feeling it,I might give it another shot though.I tried it once a few years ago when it had just gone to version 4 and didn't have any problems then.
I'm strongly considering Cubase because I'm already familiar with it,the only thing I don't like is having to use the elicenser. The other reason I'm considering Cubase is because of the VST,Note Expression. One of my favorite things about Studio One is the splitter function.