An advertisement for Support
You -- yes you reading this post -- should try out SUPPORT!
You'll get the chance to be 2-5 levels behind most of the players in the game, on 2/3 of their budget, with 5050 gold of mandatory purchases before you can choose what to build.. ..and in exchange you'll get to help your allies make some really sweet plays!
Riot, I love playing support, but there is some baseline amount of agency I need in a normal game to make the role worth playing, and it just ain't there right now.
Edit since people weren't noticing the link: Here's why I love playing support: http://boards.na.leagueoflegends.com/en/c/gameplay-balance/me7PRvfg-an-advertisement-for-support?comment=0000
while you can. They're making big changes in the pre season.
on it.
vs AP:
vs AD:

or
or
if you need in-combat MS, tenacity, or if they have many autoattackers
for cheese picks or ults
If your team is often seiging or being seiged.
If they're pretty AD-heavy and you need to be tankier to survive
If you want to go for a better late-game build, get eye instead of spooky ghosts and pick up this instead. You can throw down tormented soil and the slow will help you hit your binding.
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