Starlin-era Thanos/Captain Marvel was an awesome rivalry, made even stronger by the fact that at the end Thanos seems almost sorry his greatest foe won't be around any more so he helps ease him into death with a final brawl before they both hold hands with Death and wander into the afterlife. Brilliant
Can't get any worse than those two. Sabertooth visits every yr on Logan bday to kill a loved one. Joined the Xmen to kill everyone close to him. Slaughtered Psylocke right in front of his eyes. Every girl he has dated, Creed kills them. He isn't just a villain that is there to try and kill Logan. He wants Logan alive so that he can kill everyone that is close to him. Basically wants him to suffer for eternity. Can't think of anyone that has a villain like the way Sabertooth is to Logan.