What is the worst monster in the 5.5e Monster Manual?

I've gathered the monsters that have received the most criticism in the 5.5e MM, and I want to answer the question: Which is truly the worst? Feel free to comment the reasons for your vote. P.S this is all RAW stat blocks, no homebrew tweaks. P.P.S. This isn't a critiscm of the MM itself. The fact that I have so little options so should speak that most of the monsters are very good.

I'd have to go with Werewolf, though not because of the lack of damage immunity. It's the way they handled becoming a lycanthrope. It's just awful.

What are the criteria for being "worst"? Are we talking "not up to it's CR", "worst to deal with as a player", "most complicated to run as a DM", or something else entirely? You mention criticism, but you could criticize the artwork and fall under that umbrella.

Worst could depend on several factors:

1. Monotonous or frustrating to play against.
2. Nonsensical abilities.
3. Mechanics don't fit with lore.
4. Is too weak or powerful for its CR.
5. Is just generally an unfun monster to experience in the game.

I would say artwork doesn't count as i'm talking about the stat block, and as for the DM complexity, that's more a experience issue with the DM, unless it's really complicated, which none of the monsters in this book really are.

An example:

Werewolf could fit into 1, 2, or 3.

Noble Prodigy could fit into into 1 or 2, though more strongly.

Silver dragons could fit into 2 or 4.