I'm a Tenchufag.
2 isn't as bad as posers say, and you shouldn't skip it. The ranking system has been botched, and it has no soundtrack, two baffling decisions, but it's good otherwise.
Return from Darkness's extra content isn't enough to make up for the lack of extra shoulder buttons. Picking up bodies was also tacked on to answer to criticisms, the game isn't built around it and it makes no difference.
Other than the original 3 games, Fatal Shadows which is a sequel to 3 is pretty good and also worth playing, you can ignore every other Tenchu game.