I like all classic Mustangs, but the reason there are three 64.5-66 Mustangs in my shop is because I clearly have a favorite.
There's something about the mid-century optimism, the slightly goofy "Woody's Roundup" early 60's cowboy aesthetic that you see in the details like the way the rear bumper turns up on the ends. There's an indescribably purity to the lines created, before things got meaner with sharper creases. Plus, when it comes to the fastbacks, I absolutely love the curved nature of the roof in a 65-66. More rounded, both in profile and in the way the back glass meets the trunk. The five bigger vents... all of it is just perfection to my eye.
I think a great case can be made for the 67-68 being refined in every measurable way, but in that refinement, it lost a little bit of innocence that the original car had.
You would think that since I tend to prefer meaner, modified versions of those cars, I would lean to the newer cars, but I just don't. When I close my eyes and think "Mustang", I see and feel the gentle curves in the coves on the sides. The gills on the sides of the grille. The simple, elemental nature of the dash. All of that makes the choice super-simple for me. Maybe you can find that feeling yourself. Just know, there's not a wrong choice.