I prefer the "chameleon-camo"-style which makes you almost completely invisible while not moving, but doesn't hide you too well while moving (depending on movement speed).
The HL2-Mod Dystopia did this almost perfect. When standing still, you blend in with the environment perfectly, leaving only a hint of your silhouette which can only be spotted in bright areas. While moving, however, the camo flickers/moves, which is incredibly obvious if you are moving fast.