The most optimal defense, in my personal opinion, is a defense that uses a combination of the 4-3 Flat Zone and the 4-3 blitzes. You'll want to use rules to set up when you blitz and have roughly 3-4 of the blitz 1 and blitz 2 plays. The flat zone is decent enough against the run while smothering against the pass. The blitzes should help stuff runs and with a good secondary should not get burned too often. Then just use goal line vs goal line, and make a rule to use the Dime 2 deep against the passing sets and when you want to play a prevent defense at the end of the game while you're ahead.
