With the wind - to harass early game, to be effective all throughout the game.
Lost Soul - cooldown reduction that is not really noticeable to take.

Envenom - deals damage over time to a target.
Ranger's Ambush - refills your Q WHEN using the teleport. Does not restock Q when just using E and not teleporting with it.

Shade Form - stealth gives you another form of escape or ambush.
Unstable Poison - makes you an insane creep clearing hero.

Wailing Arrow - AOE (rather heavy) damage and silence. Silence is invaluable.
Possession - take this if you want to go pure specialist pusher

Evasive Fire - gives a boost to your chase and run capabilities
Overwhelming Affliction - gives you a 5% slow on all skills and hits on heroes. Stacks up to 5x. Lasts for the duration of Black Arrows.

Blood for Blood - sustain and damage. great for finishing or leeching life from tanks
Cold Embrace - lowers the range of W but makes the enemies vulnerable while under its effect.
Will of the Forsaken - boosts your mobility for chase or escape
Windrunner - adds another E for 2 seconds after teleporting using E. Does not add another charge if you did not teleport using E.

Fury of the Storm - bonus aoe damage every 5 seconds
Bolt of the Storm - mobility via teleport/blink
Deafening Blast - Enemies in the center of the aoe will be dealt 50% more damage and silences it for a total of 5 seconds