What cooler should I get for i5 6600, NZXT Case.?

Solution
The stock cooler is plenty, and free. It's more than enough for the 6600, and will be quiet under any real-world loads. Anything you spend on a cooler is essentially wasted money.

How about a sticker to put on your case window to obscure the stock cooler, if you don't like how it looks?
The stock cooler is plenty, and free. It's more than enough for the 6600, and will be quiet under any real-world loads. Anything you spend on a cooler is essentially wasted money.

How about a sticker to put on your case window to obscure the stock cooler, if you don't like how it looks?
 
Solution