I know it's a matter of opinion, but I think the MM is the best designed out of the three. I love how they took time and effort to flesh out the lore and history of each monster. Because to me, that's more important than any ability in a stat block because it tells you how the monster plays in the game. And how something plays in the game has a bigger impact in my games than X ability or Y power. It reinforces how monsters are more than just stat blocks, and for someone who runs their games in a living world, that's important. YMMV of course.