The best is the highest MPG you have obtained since a reset of the average. Mine shows 40.1. This is the highest I have obtained on a tank (I reset the average when I refuel). If my next reset is 42.4, it will replace the best.

As mentioned above, if you select 'reset all' it will clear the best, average, and screen. When you clear the average it should also clear the screen (I think). I cannot tell if there are two functions to the reset -- one tap clears the average -- tap and hold clears the average and screen. I guess I just press it until the average and screen clears.