I would choose Sanchin for chinkuchi development and Naihanchi for fighting applications. Wansu and Siesan would be next on my list. For those who don't know, chinkuchi literally means bone, sinew, and energy. Chinkuchi is basicaly total controll of your technique and center of gravity, focus, directing energy to different parts of your body and propper breathing.These are all things that I believe Sanchin teaches. I also believe that this is why so many Okinawan styles stress Sanchin so much. Isshin Ryu is a combination of Goju Ryu and Shorin Ryu. They way that I practice Isshin Ryu is to use the Goju Ryu Kata for chinkuchi development and the Shorin Ryu kata for fighting. I must say though If I could pick only one it would be Sanchin. I would use my basics for fighting.
It's not the quantity but the quality that counts.