As BOZ mentioned, the greatest creature of all is the... FLUMPH! FLUMPH! FLUMPH! Ahem.
I am also looking forward to the whole mess of obscure outer planar monsters that got left out of the MM, like the gehreleths/damodands, nupperibo, and the rest of the daemons.
But that may just be because I like Planescape and using fiends to wreak havoc in general.
I have also always like the necrophidius and the scarecrow, particularly when it occaisionally becomes conscious and intelligent. These constructs seem to have a great bit more flavor than the standard golems.
I don't know if the time and psionic elementals are from previous editions, but they look interesting as well.
Vegepygmies, huecuvas, and numerous other monsters are fascinatingl simply because they are so bizaare and/or twisted.