I went with Rescue Me although it was a tough call with that and the Shield. Either way both are superb programmes.
Like the poster above I'm also in the minority who thinks 24 is completely overrated. I would recommend you try
Sleeper Cell which is similar in the sense that it deals with terrorism. The major difference is the quality of writing, acting and actual three dimensional characters including the "bad guys". Another suggestion instead of watching 24 is a movie called Spartan starring Val Kilmer written by David Mamet.