Originally posted by Tondemonai
Reasons?
He is a master swordsman with history of out-dueling other master swordsmen and/or extremely fast duelists. He is also powerful in the ways of the Force with history of developing new powers such as Force-maelstrom. His powers are sufficient to overwhelm conventional defenses of another (powerful) Force-user.
In addition to the above, Palpatine somehow obtained documented records of exploits of Darth Malgus and deduced him as a role-model Sith Warrior for others (including Darth Vader) to follow, learned new stuff from him and acknowledged him as one of his strongest predecessors with virtually unparalleled performance in combat situations.
Considering melee expertise: both are excellent in this area. However, Darth Malgus is not physically constrained by body armor like Darth Vader, and is (surprisingly) agile and very acrobatic for an individual of his size.
IMO, minor edge to Darth Malgus in this area.
Considering Force powers: I find both comparable in the discipline of telekinesis but Darth Malgus have advantage in other aspects/areas; he is incredibly proficient in the use of Force Lightning and also have Force-maelstrom.
IMO, Darth Malgus can achieve a breakthrough in an exchange of Force powers with Darth Vader by virtue of potent expressions of Force Lightning and/or Force-maelstrom and/or some other talent we are not aware of.