After sampling a fair few iPhone cases, I've come to the conclusion that the official Apple iPhone cases are the best in terms of not interfering with flash. One of the main reasons for this is the fact that they're precision made to fit perfectly with the iPhone, but there is another reason too.
The ring around the camera cut outs on iPhone cases need to be black in order to get the most out of your camera. This is due to the fact that other colours interfere with how the flash sensor works, and can give you less than desirable colour tone results.
Not all unofficial iPhone cases include this feature, so you're probably best choosing a case that does, but there is one unofficial iPhone case that I know of that receives favourable reviews.
OtterBox cases are popular throughout many iPhone models, and they're designed to be extra sturdy. The most popular feature, however, is the large opening for the camera, which means the case won't interfere with the flash.
Unfortunately, the quality of the OtterBox case is reflected in its price. You can expect to pay around $60 for an OtterBox iPhone 6 case. Still, that's better than buying a cheap case that doesn't protect your phone and interferes with your camera flash.