8.5 really. They cut the last one short for budget and low viewership, but the excuse was cutting out filler time. Every season has some junky side plots, often involving Jack's super hot but awful daughter. S1 has a particularly bad amnesia plot straight out of soap operas, but they do have lots of action so its still ok at low points.

Seasons usually start slow, but it has a lot of the excitement that Game Of Thrones had because you really never know when they'll just kill off an important character. Its such a thing that in S9, the president tells Jack to stay away from his daughter because "everyone who gets close to you dies horribly".