Shiny Relicanth is really breathtaking and is definitely one of the best shiny sprites out there. I just love any sprite that has that shade of blue and Relicanth is no exception.
I've always been quite fond of Gigalith's shiny sprite as well:
His original sprite is just OK, but the shiny version makes him into a beautiful Pokemon. The coloration is near perfect and the blue suits him really well. This is definitely my favorite shiny sprite from B&W and possibly of all time!