U.S. Marshall or Secret Service, very tough choice between the 2....Secret Service would probably be a tad more stressful seeing as the president's and his advisor's well being is in your hands, but thats not all they do, like most law enforcement and protective agencies there is A LOT of red tape and paper work to go through.
Now U.S. Marshall is a bit more realistic of a job wish, still hard to get into but if this is "Dream job" then its ok
Kicking in doors and nabbing convicts and felons with good weapons and well trained team mates to back you up sounds like a good job. Probably wont be saying that when I turn 50 though....