Hmm, if I was designing the perfect cock - on me, or on a partner?
On me I'd like perfect symetry - even 6" girth from corona to the base. 9" in length - I know that is edging into too much but it's the number I've always wanted. Balls hanging 5" below my crotch, I have some swing, but more would be better. The feel of heavy balls swinging hard is.... just next level pleasure.
On a partner. A thicker skin, I prefer playing with a cock I cannot see blue & red of blood within. The bulging blood vessels makes no difference, but I just prefer a nice solid coco skin tone for the whole shaft. 4" - 7" length, thin - 2"-4" girth, a "pencil dick" with a glans that matches, not a mushroom. A foreskin that covers even when hard, nice pink/moist head. Smooth sack with 2 golf ball sized nuts inside, that has some swing to it. Not a fan of the 'speed bump' ball sack which is a lump at the base of the penis - I like a set of hanging balls that are basically their own sex organ, related but separate from the penis.