It's all kind of 'eye of the beholder,' when it comes to Mustangs. I'm actually not a fan of the '64-'68 fastbacks, and prefer the hard tops of those years - the fastbacks just look like an after-thought to me, with the boxy greenhouse looking more appropriate for the year model styling. My personal preference would have to rank them in the order of: hard top, 'vert, fastback.
The '69 fastback is probably the most iconic classic Mustang, IMHO, with the Boss 429 being the Holy Grail of coolness. The quad-headlight grille is meanest the Mustangs have ever looked, and the 'fatty' fenders just highlight the need for some fat sticky tires in the rear to scream, "Get used to seeing the taillights." The '70 Boss 302 is the coolest of the '70 models (although, a '70 Boss 429 is a rare hen's tooth, that's for sure). Then there are the '69 & '70 Shelbys - which are the coolest of the Shelbys, IMHO. The '69 - '70 fastbacks just look more like they were 'intentional' to me than the earlier fastback year models, My ranking, fastback, 'vert, hard top.
Even though I've chosen my '71 Mach 1 as my favorite Mustang, I'm also drawn to the '71-'73 'verts (thanks to seeing all the cool cars on this site).
Out of my comments above, I can't say for sure which one I'd pick today, but if I had to choose between any of the '69-'73 fastbacks or 'verts, I'd have a hard choice to make. As much as I love my Mach 1, I feel like if I had a chance to do it all over again, I might go the direction of a 'vert instead.