I can't believe only one person said Black Lagoon. That is the only action esque anime I've seen that has non-stop action.
While I liked a lot of the others, some are a bit tame (DBZ, FMA... Shaman King...?) or have lapses in the action (CANAAN, Hellsing, Trigun). Darker than Black is up and down - it is either REALLY awesome or the episode is a bit boring.
But every episode or Black Lagoon you feel like someone is seriously going to die and if there isn't action on the screen, you can feel it bubbling underneath about to explode.