The Ping ISI irons take a .355 taper tip shaft, so whatever shaft you choose needs to be taper tip. As far as the best shaft, whatever fits your game. without knowing anything about how you play, your swing speed, what you’re looking for in performance, etc. it’s difficult to make a recommendation. There are True Temper, KBS and Nippon shafts in steel that could work, and multiple brands and models of graphite that could work. If you just simply want to replace the shafts that came in the irons, the stock shafts, if you could tell me what they were I could possibly give you a shaft that would be similar.