Me too, I think preev is the most reliable and it's always up to date. However, I can't vouch for other sites because I have not used them, but I think there are many legit sites that post the closest and the most accurate information regarding its exchange value. Also, in many trading sites like coins.ph, you can actually see the current market trend and trading value of many different coins based on how much people sell it or buy it for. However, since the price of bitcoin is always changing, especially these days, the best way is to check on different sites regularly to compare to guide you in deciding on what to do with your coins.