Take out the ridiculous stunts in six, and 7, and you can easily say....

They improved with each film. Like it or not, the films got better as they went along (Especially Fast 4, and 5). You can watch all the films without Tokyo Drift, and you will see the improvement. Tokyo Drift is still the black sheep of the series, and for good reasons. You can basically watch them in the following order 1,2,4,5,6, and 7.