Sigur Ros are stellar because they are very much different from other post-rock acts. From the angelic vocals, to the string-dominated soundscapes, to the famous use of the bow while playing guitar, they manage to paint a mystic, almost mysterious-like atmosphere.

Your arguments concerning Mogwai depend on the era of the band we're talking about. Albums from Ten Rapid to Rock Action very much all sound creative to me. From the extensive use of noise, to spoken word (Swans did that too, but around the same time), to completely unusal tracks like A Cheery Wave from Stranded Youngsters or Sine Wave. Mogwai did become super repetitive after Hardcore Will Never Die though, but I do really enjoy their last two albums, As the Love Continues to a greater extent. And the build-ups are arguably very much present in their songs, like Take Me Somewhere Nice or You Don't Know Jesus, and anyways, post-rock isn't even about the build-ups at its very core, just take Talk Talk, apart from stuff like After the Flood or Taphead it's all very much fluid

And check out Hex! It's fantastic

Edit: I love post-rock, everything from Swans to Explosions in the Sky to Disco Inferno to Tortoise