[Jinxy]

The damage of auto shot is actually very good. It has 100% hit rate, which makes it better than junk arrow. It also has a cooldown though, which is the only reason you should want to use junk arrow rather than auto shot.

As for aimed shot, it only has 60% chance of doing almost as much damage as the 4 arrows of auto shot, it does however deal that damage in one shot not allowing the opponent to heal between the hits.

I have them all maxed, I will test it... soon >.<

EDIT:
Okay I did the testing of the skills, so here goes:

With 151 dex and 1065 attack rate and all skills in question maxed I got these damage results (rounded off most results and calculating).

Junk arrow - 1630 per shot - maximum potential: 6500 - 70% success rate means you are most likely to miss with 1 and hit with 3 meaning 4900 damage.
Silent shot - 1650 - Dark illusion state plays no role anymore.
Aimed shot - 1950 regular hit - 3900 when it does inflict double damage which it only has a 60% chance of.
Auto shot - 1430 per shot - 100% hit rate means you always do total damage of 5700

So considering the success rate of junk arrow auto shot is better, but if you get lucky junk arrow has the better damage potential.

If you just want to find out what's best to grind with just go with junk arrow because it has no cooldown.