Trick question, right?
Like there's a 'bad' choice on your list.
Okay, I voted for the 3913. I'd have to offer it as my opinion of a "Best all-around" choice if I were pressed to decide.
I say this owning a 3913, CS9, 4013TSW, original 4513TSW, 4040PD and a CS45. (Just listing the metal-framed models.) Also, at one time or another I've carried an issued 5903, 6906, 4013TSW, 4566TSW & 4513TSW.
I really like the 6906. A lot. It's on my list of things to look for when I can find one sitting neglected in some out-of-the-way gun shop. Preferably a later production vintage.
The CS9 is handy. No doubt about it. I often choose to carry it over the 3913 when belt holster carry isn't used.
It's just that I happen to find the 3913 to be an ideal combination of design, size/dimensions, light weight & balance ... which taken together lend themselves to an inherently accurate, controllable and 'fast-handling' defensive handgun.
I carry a 4513TSW on-duty everyday at work, but I'd happily switch to carrying a 3913TSW if it were permitted.
I'd even be happy if we still issued 6906's.
Just my thoughts.
They all work fine and are all good quality pistols.