I like autolites and the motor crafts. Just stay away from a platnium plug for a performance application. The copper plugs wear, but they conduct spark better.
If your ignition is up to it you can go with an autolite ar133. It has a cut back ground to increase the spark area shooting towards the piston (that was eloquent). Dont expect 20 hp from plugs, but do expect them to make a good combo possible.
another vote for autolites.on stock or n/a engines your not going to find magical hp with a plug change unless you had bad plugs to start with.i wouldn't fool too much with heat ranges either unless you just like experimenting(i wasted too much money and plugs)