24 has some great moments and other not so great, but, it manages to achieve moments of dramatical tension in almost all seasons apart from Season 6. In particular:

Season 3: the death of Ryan Chappelle. The interplay between Palmer resolute attitude in the affairs of State and his willingness to go dirty and compromise in his personal affairs.

Season 4: Jack Bauer tracking a suspect and staging a robbery in a gas station.

Season 5: Charles and Martha Logan. Perhaps the best actors in the entire 24 series, bringing great tension, with the emerging theme of doing evil in the name of political good. A theme revisited in the second part of Season 8.

Season 8: The second part, and the interplay between Logan, President Taylor, and Bauer.