Which site has the most accurate USDBTC pricing? I need for Bitmex margin trading, thanks in advance.
you should get bitcoin price from the same place you are trying to trade in!
if it is Bitmex then use Bitmex not some place else. the Bitmex price will be accurate for you. they are calling it XBT instead of BTC but it is the same thing. you can use their website or their API to fetch the price live from their exchange.
The platform where you perform your trading is the only important one actually. For the other prices, you can check with coinmarketcap web site and see the other prices easily.