In my mind it's always been Muller first due to his outright aggression and willingness to brawl and fight with Tintin, which he has done many times, both in The Black Island and Land of Black Gold. Muller seems much more willing to seriously hurt Tintin and there is a scene in Land of Black Gold when he knocks Tintin out with a stick as soon as he sees him. Rastopopulous is extremely evil too, of course, but his evil lies more in his drug smuggling empire and less in his direct encounters with Tintin, which are sometimes friendly, such as when he gives Tintin horses and supplies in Cigars of the Pharoah, and lets him come over to his house in The Blue Lotus. I think it's a close draw, but in my personal (and probably pretty unpopular) opinion, Muller takes the win for the arch enemy.