Title pretty much explains it all. There are a lot of services that list the exchange rate of bitcoin, and the usually vary by a few dollars, so is there one that is the most trusted?
So you are pointing with the "bitcoin exchange rate" and not with the "exchange service" like buying, selling, storing etc isn't it?
Obviously exchanges have different rates because that is business. The exchanges you are using primarily is the one you must to refer as a price basis since that is the only exchange you will use or available when converting or withdrawing your bitcoin. Even you use some exchanges with high exchange rate, coins will still landed on the exchange that is available in your country.