Before I do dare to call this album one of the best progressive rock, maybe the entire rock canon, my listening crudentials are that I am a rock buff and an open listener. My favorite musics (yes, it is okay to spell it with an "s") are classical, jazz, what are usually the higher art music. I recently (well, years ago) got into seeing rock as an art form. My answers are unprejudiced, uninfluenced, and confident. Of course, it is always personal taste. But when a certian work as an undeniable musicality no matter your taste, I feel I can recognize it.
And wouldn't you know! Angherr Shisspa is such an album!
Seriously, this is a level of musicianship and virtuosity almost never seen in rock. Period. The compositions are driving, meditative, complex. The energy unwaivering. Comparisons like to be drawn between this and Magma albums (another gooden) but I can only compare this to one album I've ever heard (and I've heard 'em all) and that is Dun's only album Eros (can you say masterpiece?).
This album for me gets put in a rank of the greatest fusion albums, of which I use the term loosely: to me it means not just jazz-rock, but all music which obviously combines more than one genre into a "supergenre", that is fusion. These great albums are: Blood Sweat and Tears - Child is Father to the Man, System of a Down - Mezmerize+Hypnotize, Frank Zappa - Absolutely Free, Henry Cow - Unrest, Spring Heel Jack - Disappeared, SBB - Memento z Banalnym tryptykiem, Godspeed You Balck Emperor! - Lift Your Skinny Fists..., Mahavisnu's Inner Mounting Flame, Bitches Brew, Eros, and this. Some of these might not even make my top 30, but they are the prime examples of mastery over many genres. (Obviously, the "great albums" could probably be cited as these kinds too: Trout Mask Replica has a blues influence and The Whie Album is all over the map, but since those are indeed primarily influences and not obvious physical discrepancies I leave them out for the sake of organization!)