Straight up saber brawl would go to Palpatine. He was able to shred three Jedi masters, broke a bit of a sweat with Windu, and dueled Yoda to a draw. Kreia? Combat is her weak point. She's mediocre to awful with a saber unless she's TK-ing it.
Scheming ability: Tough call, but I hand it to Kreia. The main problem is that Palpatine's stunts got found out eventually. Meanwhile, while we know she was playing EVERYONE in K2 like cheap flutes, Exile included, we can only guess at the REST of what that misplaced Reverend Mother managed to do under everyone's nose. We still don't know her true identity! Ever notice she was the ONLY one in K2 who seemed to know anything about the "True Sith?" Was she one of the ones who set up the Mandalorians? Did she set events in motion that led Revan down the Dark Path and right to the Star Forge? The fact we don't know, and the fact she worked so quietly that no one either noticed or could do anything about it, is definitely impressive.
Sith Skillz: Considering the amount of knowledge and techniques available to Palpatine was probably limited big time by Bane's Rule of Two, as compared with Kreia's encyclopedic knowledge as both Trayus's guardian and a Jedi Archivist? She hands him his sorry butt in this category.
Ambition: Palpatine wanted the Galaxy, like any meglomaniac. Kreia essentially wanted to kill God. Palpatine succeeded in his ambition, but definitely had his sights set lower than Madame Kreia.