Bitcoin Is the well known crypto-currency and most the time Number one market-cap holding by Bitcoin.All the other Crypto-currencies are formed after the bitoins success you can see current market value and the other market details from above chart. currently BTC trading at all the crypto markets.main pair for BTC is USD, But USD means not actual USD it is USD pegged Currency called USDT(value is not changing always keep it as 1USD=1USDT)

But these days lots of USDT  alternatives are coming to the market (TUSD,PAX,GUSD,nUSD). BTC trading with those at several crypto exchanges