Well, looking at it from a crassly commercial viewpoint: Death Note, definitely. Bleach and Naruto are both 300+ volume behemoths as manga go so you'd have to be prepared to invest quite a lot if you actually want to get anywhere with them. Death Note is of controlled length, doesn't over-size its cast to the point of ridicule and is just - in my opinion - better in terms of story and character development to boot.
If you're more into huge energy attacks and long-winded battle scenes, though, then I'd say Bleach. Naruto has waaaay too much Uchiha angst and general shallowness to it. :\
That being said, I personally find both Nabari no Ou and Fairy Tail to be far superior alternatives to all of these, but whatever floats your boat. *shrug*