World Eaters are actually a lot of fun to play. There is a lot more diversity available than you might think. Sure, most of it is based in close combat charges, but it takes a lot of good tactics to keep your men from being blown out of the game before they reach the ranks.
I'd question Nurgle armies, except that I know they are great. Plague Marines in close combat are fierce opponents. Nurgle armies are very much like Khorne armies in that regard.
I think that it is silly to argue about "who is best"...because it all comes down to the general, the army selected, and luck.
I recommend that we ban all sort of discussion from hence forth.