guys i know you are new to this game, so let me tell you this:
C'thun was fixed, not nerfed...
there was the problem of unfinished/untested content back then... so get your facts right.
i would still say C'thun because of the coordination you need in this fight / or probably Entropius pre nerf
anyway, stuff was different back then, guilds had to find their own tactics and there was no addon support like nowadays