Depends on what best means.
Creed has the most depth, is the most intelligent, the most charming, and sets Rocky up to become the Italian Stallion. Only drawback is he's basically just Ali which is less than original.
Clubber is vicious, confident, hungry, beat all the top contenders, and has a truly unique identity between his appearance and speech. Not to mention he's right about Rocko and knocks him out, something no one else could do. He goes out like a chump though, which showed that it was really just Rocky beating himself in the first fight.
Drago is the most intimidating by far. A cold killer with an evil empire behind him. Coolest quotes, biggest danger, just the ultimate villain. He isn't very active as a participant in the story though, he just sort of lets others control him like a puppet which works for the story but it takes away some villain points.
Gunn is the slimiest. The movie is a dumpster fire and we don't get a true fight. He gets the benefit of having an actual story arch with character progression like Creed, but the story is uninspired and poorly told. He would have been a better villain if the movie itself was better, there was potential.
Dixon was the definition of meh. He was pretty reasonably annoyed to even have people consider the fight anything other than a first round knockout, so not really evil but a world champ with respect for himself, then fails to even beat the 60 year old, one eyed, brain damaged guy with nearly 25 years of ring rust.
I didn't watch the Creed movies yet. I plan to.
I would say from an artistic standpoint that Creed is the best, Drago is the best as an obstacle for Rocky to overcome, and Clubber is a hybrid with traits of the other two, dangerous in mind and power, hungry, and actively seeking to hurt Rocky. I enjoy Creed as a character more, I enjoy Drago as a villain the most, and Clubber is the coolest.