Varus is an amazing carry and i feel he is underplayed considering his potential. Most carries rely solely on basic attacks however, Varus can also take of 15% of an enemies health as magic damage aswell as the damage of his other abilities which i feel puts him ahead of other carries.

Many people who play varus max their Piercing Arrow first as it maximizes his poking ability. I think that this is a bad choice on Varus. It allows opponents to wait for the player to waste the cooldown and then initiate on him with a large source of his damage missing. I always max my Blighted Quiver first. It means that your basic attacks will do a little more damage than other carries plus you can use it with all of your abilities. At level 9, you can potentially take off 45% of the enemies health just with your Blighted Quiver. Obviously since you need to basic attack to get each stack on the target, it's unlikely for you to get 3 stacks of 3 detonated on them but my point still stands. Maxing your Blighted Quiver will allow you to 1v1 pretty much any carry except Sivir who can block the damage from both your Blighted Quiver and whatever ability you used to detonate it with. This is a problem for all Varus players no matter what spell you max first.