Cubase is the most user friendly while probably having the least serious instruments. It is also the most advanced.
Logic has by far the best instruments, effects, presets, track presets, etc. It has been ahead of the game in that dept for 5 years or so, and nothing is trying to compete with that. Logic is also the least user friendly of all the popular sequencers. Can't have your cake and eat it too.
Sonar is kind of OK. I never could warm up to it. It has always felt like shareware. Clunky, and jerky. One thing Sonar and Logic have in common is the audio dropout. Both will suddenly stop audio playback for no reason at all. Both have a built in method for getting the audio engine started back. It's weird. Neither have solved this problem to date. That doesn't surprise me about Sonar, but while Logic suffers from pointless dropouts, it also has the smoothest audio engine of any sequencer. In other words, you can add a VST Instrument, during playback and the audio won't skip a beat, nothing. It's amazing really. You can add a track preset. That's a VST instrument with like 5 effects that load up automatically, during audio playback, and the audio won't skip a beat. Do that with Cubase and you'll wonder if your computer is going to explode. I'm told that this is because of Cubase's superior implementation of automatic plugin delay compensation which by the way works for both internal and external instruments and fx.
Cubase and Logic both have very powerful midi, but Cubase might have an edge for some of it's more advanced midi tools. Sonar is also pretty good.
Logic has horrible audio editing. You'll be surprised just how weak it is. Cubase is arguably the very best audio editor. The kind of powerful audio editor that makes you not need an external editor. It's that serious. Sonar is not bad but not near Cubase.
Cubase is my favorite but I would kill for all of Logic's instruments, fx, presets, etc, etc. That content is just amazing.
Oh yeah, IMHO.