What fuel is best?

I had a 1970 Dart with 225 slant 6 a few years ago and I was never sure what fuel to really put in it. I was using 87, 89, 93, and any mix of two and some times an octane booster. The car ended up breaking all its compression rings on one cylinder. And would never idle again.

What fuel should I use in my next Dart? It does not have hardened valve seats so I will use a lead substitute.