One stat I use when evaluating Offenses is % of 1st Down Conversions made on 3rd down(lower better). Or the inverse % of 1st Down Conversions made on 1st and 2nd down(higher better). Take all 1st Down Converstions by an offense(looking at passing only) and divide that into the sum of passing 1st down conversions made on 1st and 2nd down. 1st down conversions include TDs and Penalty awarded 1st downs.

As of week 13 of last season Goff led the league with 76.3% of his first down conversions(by passing) coming on 1st or 2nd down. Newton was 2nd at 75.7% and Brady 3rd at 75.3%. The NFL average was 70.2%.

When you never see a 3rd down you can never fail to convert on 3rd down. For me, this stat is one of the key indicators of a successful offense.

EPA is also very telling for offense and specific situations. Also, Yards Per Route Run is a very good indicator of the quality of receiving target.