Momo Sissoko has unbelievable potential based on his agility and physique. However, he better learn some ball skills pretty past becuase his lack of vision when in possession is definetely a drawback.
Patrick Viera not only had outstanding defensive qualities as young lanky kid, but his ability with distribution was already evident. I highly doubt Momo Sissoko will ever develop into all-round Viera type, despite the similarities in physique, and I think he will probably end up in the Rino type of player.
With regards to best CM in the world ATM, I think it is a shootout between Gerrard, Pirlo and Makelele. They all have their own attributes, but in a right system, I feel three players are the ones that will express themselves best.
And Lampard is by far the most overrated player in the world. I always admired his dedication, workrate on and off the pitch, and personal qualities, but he simply doesnt have that "thing" which players like Kaka, Gerrard, Pirlo, Ballack et al possess. As for the most underrated, I say Owen Hargreaves and Senna. Iniesta is up there as well.