In a versus fight? Ragnos wins.
As far as manipulating midichlorians, how is that necessarily a unique ability? Sith Alchemy created great monstrosities of the dark side, would that not involve midi-chlorians?
You have Sidious' rumor that he could stop death, that was never once established. In fact, since he was using it to attempt to sway Anakin, who feared death, it seems more than likely that he didn't develop a method for it.
Outside of Palpatine's word or a line in the NEC, there's nothing on Plagueis, he's more ambiguous than Ragnos -- whom we know at least to possess terrifying power.
As far as manipulating life, I find what Sadow did to be far more impressive than Plagueis.