In terms of power and ability to easily increase power ... 03-04 Cobra.
Otherwise - a 60s fastback or S197 GT 500 ... 74-98 models are very bland in comparison.
My sister's boyfriend in 1993 bought a new Saleen. Black with gold accents. That car was pretty amazing. He actually bought a Black with Silver, but the lot jock smacked into a wall with it pulling out of the prep bay and he took the gold accented one instead.
My dads 1967 GT500 with 2 x 4bbl 427 medium riser side oiler, Toploader 4 speed and 3.89 detroit locker 31 spine 9" rear.
He bought it used IIRC in 1969 off a used car lot. The car lot owner thought it was just a really poorly running Mustang.
My dad bought it and immediately filled it up with Sunoco 260 and said it cleared up immediately. It was one of the meanest running cars in Indianapolis in the early 70's
He used to put it into alot of car shows and won a number of trophies.
If you can find a back issue of IIRC Hot Rod Magazine where they had a Mustang II on the cover and the inset picture is a dark blue '67 GT500 with the driver having dark curly afro like hair and sun glasses thats my dads car and the driver is my dad.
He sold it around 1974 when the Toploader died. Apparently back then getting parts for a Toploader 4 speed was almost impossible.
Some old guy bought the car and was suppsoed to call my dad and sell it back to him if he ever decided to get rid of it.
The old guy kept it for a few years and sold it to some kid who totalled it against a telephone pole same week he bought it.
My dad still kicks himself in the butt daily for selling it and wishes he had kept ahold of it. He still has all the paperwork, pics, documentation on it.