Really, it all depends on your playstyle.
Although...
...you really should go for the "Venom Arrow" morph, because the casting interrupt at range is nothing to sneeze at.
...while "Volley" or "Arrow Spray" and its morphs are vital if you neet someone with projectile reflection like DKs and some monsters. I suggest slotting at least one of those, just in case.
Everything else - as you prefer. Much depoends on your playstyle... sneaky and spipe-ish? Then you may want "Snipe" for that, uh... sniping? Or more "gun and run", then avoid that one and go for something fast and in-your-face. And so on...
Hail is best for bosses who don't move around much, and arrow spray is good for mobs. Snipe is biggest single target damage, and poison arrow is a decent one, I found the interrupt morph most useful. Level them all up then take your pick depending on what you're doing.