I'm going with Krayt. If Plagueis does have a force-augmentation advantage it's negated by Krayt's far superior skill with a saber. Plagueis's piss-poor attitude towards dueling or physical combat in general likely means he rarely honed his technique after his younger years, similar to Palpatine.
And then there's feats which are basically non-existent for Hego aside from getting his defenses breached by Venamis and calling upon the force to save him.
I mean sure, Plagueis never showed interest in lightsaber combat, but I think that's taken too far usually. He still made sure Sidious was trained to use a variety of weapons for a variety of situations, and Tenebrous called Plagueis a master of the art. He also showed a lot of cunning in how he dealt with a combatant trained specifically to counter his form.
I'm not arguing for Plagueis beating Krayt, I'm relatively undecidied, but thought I'd just add that in for context.