Fran Tarkenton's Pro Football was a playable yet highly detailed simulation of pro football in the days before the NFL and AFL consolidated. (Tarkenton was a New York Giant quarterback at the time – before he led the Vikings to three Super Bowls.)
The game was advanced for the time, and very much a simulation. The game used 23 offensive basic plays and 8 defensive lines. Unlike some contemporaries and many lesser football games, the proper defense will limit the damage of a great roll. Despite the limited number of offensive plays, there are plenty of choices since the plays can showcase feature positions. Also, beyond the 8 defensive setups, the defensive player can also have the choice of which player to 'key' on. Of course, double-covering the wrong receiver will allow a lesser roll to have a higher result.
The true advancement for the game system was the yearly 'rating' of each team on their defensive and offensive line in each of these plays. Thus, when you played two teams, there was a die roll modifier for each play based on each defense.
The clock functions were also very good, allowing a true 'two minute drill' and the use of real game strategies.
Fran Tarkenton's Pro Football is a great time capsule with a look at the teams in the 1960s, before they merged to become the NFC and AFC.