For me, I like the whole package. Good colour art, readable, useable Statblocks, and most importantly, a reason why I'd use it in my game. This may be because of a quirky original ability, it may be because I want to get a reaction from the group by saying "you see THIS in front of you" as I hold up the illustration, or it may be because it fills a niche that I want in my game (or more to the point that I put into my game because it is so good).
Getting back to the package thing, I suppose what I'm talking about there is that in terms of verisimilitude, the monster works. It doesn't just come across as a collection of interesting parts, it comes across as a whole.
For me (not including the core Monster Manual), out of the additional books by WoTC, I would say the highwater mark was set by MMIII. Yes the mistakes in it were many, but in terms of flavour and the "you must put me in your game" factor, there was a lot in there. Maybe it was the great artwork that did it for me? I'm not sure but concept-wise, I just loved this one.
Best Regards
Herremann the Wise