which classical / cool monsters should a player have fought against?

[Phaederkiel]

Let us compile a list :)

format:

Monster Name
CR as it is in the books / CR as it probably should be, if it differs
Book, Page
Why it is classical / cool in ways of flavour
Why it is classical / cool in ways of mechanics

[Phaederkiel]

Assassin Vine
cr3, possibly cr4ish
srd: http://www.dandwiki.com/wiki/SRD:Assassin_Vine

I think that the flavour of a plant collecting its own fertilizer is really good.

Mechanically, I like that the Victim has quite some time to realize the predicament he is in, since the Vine kills quite slowly. I do not like its ability to move at all, and will always wonder why a Plant has low-light vision.
And it has the built-in ability to cast a 7th lvl spell.