Darth Tyranus vs. Darth Maul and Obi-Wan -
Tyranus is proficient and skilled enough to beat Obi-Wan and Anakin, simultaneously. I believe a battle with Maul and Obi-Wan would end the same way, if it is true that he threw the fight in ROTS. He will win, through superior bladeskill and Force power. If not, he will lose. Regardless, considering Maul's ability to handle multiple opponents, the Count must be on his guard. If he is, he will win.
Darth Sidious vs Master Yoda -
In my opinion, these two are the most powerful of the Dark and Light sides, respectively. They are equals to the point where neither will crush the other (though it is of popular opinion that Yoda would crush Sidious). Sidious has a slight fear of Yoda (which is warranted, considering Yoda's ancient lifespan, his knowledge, and attunement to the Dark Side, which Sidious has never been confronted with before). And Sidious has the advantage of offensive Force powers. The Star Wars site claims that Sidious was too powerful for Yoda to defeat, but the movie depicted that both fought to a stalemate - despite Yoda's disadvantage and Sidious's fear. In the end, however, Yoda will win. Just barely.
However. Tyranus will finish his duel much quicker than Yoda with this one. If Tyranus dispatches his opponents quick, he will team up with Sidious to battle Yoda. Yoda will then be dramatically overpowered, and lose, unable to fend off the attacks of the most powerful Dark Side user and his almost-equally deadly apprentices.