Fresh Cream
Fresh Cream, released in 1966, must be the one of the first heavy rock albums and inspired progressive rock too. NSU and Spoonful remain among Cream's best tracks. Although Mountain (the album) came three years later, Leslie West was perfecting his style and needed Mountain (the band). Later, Nantucket Sleighride would certainly give Cream a run for their money. I voted for Fresh Cream because it is a great album and a landmark.
Cream by a whisker
^^Slowhand is the only solo Clapton album I listen to at all. I'm not familiar with Jack Bruce's solo work so can't comment on it, but he is definitely a better singer than Clapton.