Krayt vs Sidious
Well seeing as Darth Krayt is really Jacen Solo and therefore a descendant of the chosen one his Force and therfore his Sith abilities would be much more powerful. He also over 100 years old by the time of the legacy series so he has more experience's then sidious... Look what Jacen learned in the first 20 years about using the Force. Imagine 100 years of knowledge?
Krayt is NOT Jacen. Ostrander has confirmed. On top of that, Krayt was born (under another name of course) AFTER the Yuuzhan Vong War. Jacen was already in his 30's.
And that the Skywalkers have appeared in the Legacy comics is a completely different story than Luke Skywalker and Ben Skywalker being in the comic. We've had Kol, Luke's grandson, and Cade, Kol's son.
Anyways, I'm going to say that Krayt would lose to Sidious in a battle of the Force and probably in a battle of lightsabers. Krayt is a newbie Sith, self-trained, and while he appears to be powerful and so on, I don't think he'll present the same threat that Sidious was.