McCain even had an appearance as a non-speaking extra in season 5. He just walks into the frame after a commercial break, hands one of the main characters a folder and walks out, but with a barely contained grin of a little kid getting to throw out the first pitch at his favorite team's home opener. Scalia actually, in a public debate about torture, defended interrogation practices by citing how Jack Bauer used torture to save Los Angeles.

Critically speaking it peaked in 2, as most TV shows do. From a fan perspective, watching at the time, 4 & 5 were high points. I enjoyed 6, but was in a general minority on that one, as it resulted in a mini-reboot, taking the show out of LA and to DC for season 7, and then NY for season 8. I thought 7 was uneven (and having to wait a whole extra year thanks to the writer's strike didn't help) but 8 was a fine swan song. By that point, people were more fans of Jack & Chloe than fans of "24", and more willing to laugh off the absurdities and plot stuff that were forced on the show by the format and premise.

I would recommend 1,2,4 & 5, to watch the rest if you liked what you saw, and you probably don't need to see a preceding season to enjoy one, although you will definitely be spoiled for surprises and plot twists if you watch a later season before a former. For all intents and purposes, BTW, no character is safe, ever, except for Jack and maybe Chloe. Season 3, although my least favorite, does have some character stuff that carries on into the next season, though you could skip it if you really hated it, and I think my dislike might have had to do with that being the first season I tried to watch as it came out, and that was also the last one to air on a traditional TV fall to spring schedule, which long breaks in the middle of the season. Starting in Season 4, they aired it continuously, every week, with the occasional double episode night, so they were able to maintain the pace and keep the viewers hungry. So it might be that in hindsight, Season Three was the last one with some ongoing problems (like Jack's daughter, Kim, being a regular character, and less interesting love interests and supporting characters), so it was the season at which point my patience with those minor elements was exhausted, and then the next one was so good by comparison that it tainted 3 in hindsight. My recommendation would be to watch them all straight through, but the four seasons I mentioned would be the high point (I thought 2,4 & 5 were all better than 1).

If TV shows were "Game of Thrones" characters, "24" would be the Hound. It's not always pretty, and sometimes morally appalling, but it WILL fuck your shit up if you mess with it.