When I lived in London Guinness was my main tipple so I researched this subject extensively. I realise Guinness should be the same everywhere but it possibly reflects badly on London’s publicans that the quality of what you got served up was so variable. I think they brought out the ice cold stuff so that you couldn’t taste how bad crap Guinness actually was.
The Coach and Horses in Covent Garden was the place I was going to suggest. Their Guinness, which used to get shipped in from Dublin, when most places were supplied by Park Royal, was always beautifully kept and served, and went down a treat with their salt beef sandwiches.
There was also a Porterhouse(?) which was a kind of brewpub/microbrewery that sold its own stouts in one of the streets between Covent Garden market and the Strand. No idea if it’s still there. There was also one of them in Dublin.
The best pint of Guinness I can remember having in London was in an Irish pub down Wandsworth/Earlsfield way called Tir Na nOg. Bit of a schlep from Kings Cross though.
The best stout full stop I can remember having was a seasonal offering from the Ringwood Brewery in a pub in Lymington in Hampshire. Utterly delicious.