The final episode of Game of Thrones had 19.3m live viewers on HBO in the US

"Despite the lukewarm reception to the eighth and final season, the last ever episode of Game of Thrones drew19.3 million viewers for HBO in the US on May 19th, making it the most-watched live broadcast in the network’s history
Season eight of the fantasy behemoth averaged 44.2 million viewers per episode when accounting for catch-up viewing.
Prior to Game of Thrones, HBO’s record audience belonged to the finale of The Sopranos which aired way back in 2007."
Source:  Advanced TV, 21st May 2019