This was done in PeerHub and Steemfiles. Both of these closed unfortunately. What I see is in the marketcap of a all the coins, the "stable-coins" are like 2% of of the combined marketcap.

Yet, even though Tether is bound to what we mostly use as a unit-of-account, Bitcoin remains accepted in many more places than any other crypto-currency and Tether just doesn't have that penetration. The combination the usefulness of being a peer-to-peer crypto-currency and the network effects of being the first such currency, has made Bitcoin the king of coins.