Before I start, disclaimer: I'm only Silver V and only been playing since Dec 2013.
That said, My favorite champ is by far Quinn: she is really versatile, and because she isn't meta alot of the lower elo people I play with dont know how to deal with her kit.
Anyways, the biggest thing I have learned so far is cookie cutter builds only work if you are ahead. You NEED to adapt to the situation. Enemy team have 3 tanks? Welp better prioritize that BotRK over IE. Whole enemy team squishy but bursty? BT shield should help me survive. I spend the loading screen time looking at the champs and try to create 3 item builds in my head based on who gets fed so I don't have to figure it out in the fountain while the enemy takes that mid inhib.
That said, there are a few things that are ALMOST **NEVER ** a good idea:
- Boots first time back over AD/AP? Unless you can dodge every skillshot thrown at you (a statistical improbability) probably not the best idea. Execptions would be laning against that Thresh or Blitzcrank that arent leading the grabs.
2)AS over AD. Unless you are an stack based champ, like Vayne, Varus, Twitch, MAXING YOUR STACKS FIRST, AD is almost always more helpful. SS/PD over IE? Bad idea as an ADC. Sure you hit them more often, but you are just tickling them. A little AS may help you farm, but not as much as that shiny BF Sword.
TL:DR Know the champ you are playing and build for the match, not MOBAfire.