Great call on Surrealistic Pillow (Grace Slick didn't even appear on the first one--and Barry's article).

My choices (from the same era):

Buffalo Springfield Again--far superior to the debut

Neil Young, Everybody Knows This Is Nowhere--ditto

Big Brother and the Holding Company, Cheap Thrills--Landmark Joplin

Nico, The Marble Index--Unique and harroiwing

Gene Clark, White Light (I'm counting Echoes / with the Gosdin Brothers as a first album and classifying the Dillard and Clark album as not a solo work, though it would be a strong second album anyway)

Soft Machine, Volume 2--a major departure, but after which they headed into a new direction again, always innovating

Robert Wyatt, Rock Bottom (best second album on this list)

Kevin Ayers, Shooting at the Moon

Caravan, If I Could Do It All Over Again, I'd Do It All Over You--A prog classic, but the third album, Land of Grey and Pink, is their high water mark