uh, whats yours? I know vol. II the best and like it more than most subsequent stuff...never was a big fan of Elton Dean's horn. The first two Soft Machines are crucial along with most of Robert Wyatt's solo stuff. I'm also a big fan of Gong - the Machine's guitarist, Daevid Allen, next band.
Soft Machine, esp. on their first two, have the trippiness, cohesion, technical virtuosity, of Can but inject some quirky British humour into the mix. Not as consistently strong as Can's best work but still seem to be suprisingly underappreciated and unknown.
I don't know if I would put Can and Soft Machine on the same playing field. Can was not nearly as interested in portraying that avant-jazz feel, nor was Soft Machine as unstructured in their compositions. I love Can, but I think the comparison is apples and oranges.
My favorite is by far Fourth. Not sure why - the compositons and saxophone improvs are entrancing!