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.