I guess the answer to this question is pretty straight forward: Bitcoin.
Almost all coins pegged it to bitcoin simply because its the most tradeable coins out there so common sense dictates that its the best pair.
We also have Ethereum, for me it is still the second coin and trading volume is still up although it has suffered worst than bitcoin in this bearish trend. Otherwise trading pair is worthless to me, specially stable coins, just saying.
Maybe will be good if each exchanger make their own good coin like Binance maybe, CMIIW. So beside get fees from bitcoin pair, they can get more income and users can get more reward maybe if they really creative to create system.