I feel like the reason Ghestis works well as a villain is because not only is he incredibly striking from a design standpoint compared to the other evil team leaders (that cloak, yeowzas!), he also raises the bar by being a genuine threat to Unova.
From giant warmongering castles to, uhh, giant warmongering ships, he's got an arsenal, too. With a platoon of trained goons to follow his every whim to boot as well. His presence in both BW and B2W2 is always hinted at in some way, shape, or form to add, making him constantly loom over the player's goals. He's always present, whether you know it or not, and the game makes sure that this is clear.
Last but not least, he attempted to utilize his OWN SON in HIS PLOT to dominate the world, to corrupt him into a spitting image of himself through distorting his naive ideals on the world. He's a manipulative, cunning warlord who knows how to bend and shape the dreams of the populus to his ideas.
And we love him and his story for it.
