I have been trying to get into the Mythos beta (still no luck >_<) and from what I have seen so far the game seems pretty decent and the best part is its free. If you had to purchase it I would have probably had no interest in Mythos.
Hellgate brought forth some interesting ideas, but was executed terribly. Mythos looks nice, but as many have pointed out, it seems solidly grounded as a D2 clone.
Overall, the Diablo series to date has been rock solid. While the original was a little one dimensional, it set the tone, and really paved the way for the second. Diablo 2 expanded in monumental ways while remaining, at it's core, simple. And as to LOD, there is a reason many gaming sites consider it the best game expansion ever.