I can't say anything about the latter, but when comparing Regulus to Makima, yeah Makima is the better villain.
She was pretty much the main villain in Chainsaw Man Part 1.
Regulus was only a main villain during Arc 5 and he wasn't even the only main villain in that arc and also for many, Regulus was handled a bit more better in the light novel compared to the anime.
Regardless I do still believe Regulus was a good villain but he is no Makima.
Oh, I agree Regulus is a good villain, but it is also true that his hot-tempered behavior is too annoying for him to build a great villain cred for manga/anime audience.
When I think of best anime/manga villains that are commonly accepted as such, aka the likes of Johann, Dio, Makima, and Griffith, they share the trait of being mostly calm and collected. In comparison, Regulus is really bratty. This, I am certain, does not help with his reputation, particularly in this contest.