There is no true best plot to settle the first city on, some position may provide a slow start but become a really good city plot after an era or two, while others go the opposite way.
Different mapscripts value plots differently for starting position, some take into account what bonuses may be revealed during the starting era and the next era when it adds value to the plots while others may not consider unrevealed resources at all.
The AI suggestion is usually a very good suggestion, but this suggestion does not take into account what bonuses may be revealed later.