I haven't tried the aftermarket seats like Mustang or Corbin, although I hear great reviews.
I have tried about every version of the HD seats. I've had the stock seat, a Sundowner, the Hammock, Road Zepplin and Tall Boy seats on my various Touring models. I've tried the gel pads, Mesh cooling pads and sheepskin covers.
The final solution for me was a Sundowner seat with no pads or covers on it. Short rides or long days, it was simply the most comfortable for me.
I highly recommend them.
Bo