The best exercise for your cock is sex. After all that's what it's designed to do. During sex, and I'm including masturbation here, you Kegel exercicse your pc muscles without thinking about it, plus your erection is exercised.
I also do a penis enlargement (pe) routine. Not strict like some guys - I don't want it to take ove my life - but I do some cock exercises most days. But the single most useful thing you can do for the health of your cock is to look after your health in general. I mean get enough sleep and exercise, eat a good diet, don't smoke, don't drink much alcohol, make sure you hydrate sensibly, etc. If you don't look after your body, you can't expect your cock to give it's best.