Well, it would depend a lot by how each person thinks what makes a good villain, but I personally thought Rappa had better depiction as an individual character than Muscular.
So, here’re reasons why. First, I personally thought Rappa felt more ‘human’ than muscular. Let’s look at Muscular first. When muscular encountered Deku at the>! training camp, he had exhibited pretty crude personality. He never really had an “Deep” purpose why he was committing homicide against people. All of his motivation was fitting people and beating them to death. For him, fighting was just an entertainment, which had nothing deep. Even when Izuku encountered him again at the dark hero arc, he never shown any change in his personality. (It is shown by Izuku asking to Muscular why he became a villain. Muscular just hold him that he wanted to live an regretless life, that he could have all he fun he wants.!< However, Rappa was bit different from him. When Rappa first encountered Kirishima and Fatgum at the>! middle of Overhaul arc, he just looked like a bloodthirsty fighting addict, which have not much difference with muscular. However, when I red trough the episode carefully, I learned that he is more than just an maniac. First, he knew how to take fight seriously, not just for fun. Even though he was an antagonist, he was able to show his own type of respect at Kirishima’s will, even though it was still in very aggressive way. Also, he had shown that he have his own faith as an fighter, and some kind of antihero, by being an only member of the Overhaul’s Yakuza group, who did not became Overhaul’s dog.!< Like this, Rappa was more than just a barricade of the heroes. Instead he was an pretty solid character who had his own faith, and wild, but also pretty appealing personality.
Second, unlike Rappa, Muscular had “Aura loss moment.” >!As I mentioned before, Deku meets Muscular again at the middle of the city. Muscular had never got weaker than before. (Actually, I had saw some folks saying that he may got stronger.) However, Deku had totally busted him, only using few minutes maximum., making Muscular as a tool which had got spent to show how strong Izuku had before.!< Even though it is obviously true that Muscular is still objectively powerful, and Rappa is weaker than him, I think depictions like this made muscular less threatening than he used to be.
I may be wrong, but this was my opinion about two characters. You can point out my potentially wrong points at the comment section!