My suggestion would be The Bear in Spring Street - literally just a couple of minutes' walk from Paddington station. Up the ramp beside the Elizabeth Line entrance, cross Praed Street, and it's there on the left.
The pub is in the Good Beer Guide, it has six changing real ales from small unusual breweries, twenty-nine keg beers, 200+ bottled beers. There is a large beer menu to help you decide.
If you think this sounds like beer heaven, I'd say you were right.
"The Bear" is not to be confused with "The Mad Bishop & Bear" actually on the station as mentioned by Haywain above. I like both, but the Bear has more interesting beer by far!