It depends what you want.
If you want the best bang for the buck, 11-14. They are great cars, a good chassis and not too expensive.
If you want something completely different than any other Mustang every produced, go with the 15. They are a hell of a car. Great setup, more of a sports car than a pony car IMO. They handle great, same great 5.0L, better ride, better control, more refined interior. Fit an finish is the benchmark for Mustangs. The downfall is the price. You are likely to spend 30-40K but it should hold its value a bit better than the S197 platform.
I was just having this convo with another member here. We were talking about the differences in the cars thru the years. Fox to SN95 wasn't a crazy jump in R&D, from the SN95 to the S197 was a HUGE improvement. S197 to S550 is almost equally as big of a jump for improvement.