Piece of Time is fairly technical for its time but not all that progressive. Still many other albums from around the same time or earlier that were way further out there (the first three Mekong Delta albums and both Watchtower albums for starters). UP is the busiest album and probably has the highest riff-count not to mention a billion drum fills a second, but the riffs are individually ordinary, it's just the sheer left-turniness that makes it stand out. Elements is catchier and has more obvious hooks, yet at the same time the songwriting itself is much more layered and subtle, and builds more progressively.