hard to nail down just a few.
Artwise, I love the gnoll (released awhile ago in a preview I know), the myconids, the manes demon and spined devil, merrow, black pudding illio, roper, and zombies.
Content wise I like the realm effects of dragons and some other creatures. I like how many monsters have plot hooks in the flavor text. Many times I caught myself thinking, "Man, I could totally build an adventure around this!"
Basically, there was a LOT of stuff there that sparked my imagination on how to incorporate the monster into an adventure in fun and creative ways. Love it.