I actually think they can take them all. In 4 Kit Fisto and Agen get blitzed by either Marek or Kenobi, so Windu is kind of gets outnumbered.
And in 5 right at beginning Marek Force crushes Grevious. While Kenobi fends of Ventress, Marek either Force crushes or kills with lightsaber Dooku, doesn't really matter. And then either Marek honors Kenobi's fight and simply wait or just another Force crush.
I know, sounds very biased but come on. Marek drilled both Force and saber skills from childhood to perfection and his power is above any of them. I would say only Windu is adequate opponent for him out of all but his mates are rather too weak.
Which is why he is more likely to lose. There can't be big difference in combat as Marek put many years into his training in sabers and as Sith he drilled it.
Yet in lightsaber combat Dooku was an almost unparallelled prodigy. Galen is good, but recall that he had problems against Vader and Shaak Ti in that department.
Game over at 4. Windu crushes either in saber combat and has the prowess to defend himself well enough with the force to get in and kill either as well, and that's not even bringing in the other two backing Windu up.
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The point of Marek was kind of to be an uber force wielder. His force powers were pretty extreme. Even as a jedi he used force lighting when he fought the emperor. So in pure force power I think he would topple most of the people on this list. According to the books he knew many different styles of Lightsaber fighting including Obi-Wan's in fact iirc when he was "pretending" to be a jedi that is the style he used for the most part. He would fall back on his sith styles but he was also versed in jedi fighting. As far as him beating duku I am not sure if he could match him in a duel but I don't think he would play fair either....Im betting he would use the force to over power him if he felt like he was losing.
Instead of lowballing Vader's combat prowess would be appriciated, if someone provided any canon quotes that he became weaker fighter since RotS. All we have is Luceno novel, where he overcomes his limitations and takes advantage of his greater strength.