Who is more of a villain: Luke O'Neil in The Thorn Birds or Rasheed in A Thousand Splendid Suns?

While neither of these men have much to recommend them, I would argue Rasheed is the more villainous of the two.
Although Luke O'Neill takes undue advantage of his wife, Meggie, he never strikes her. On the other hand, Rasheed physically assaults both his wives, Mariam and Laila, without compunction when he wants to get his way. A commonality between the two men is that both believe in the thorough infallibility and authority of the husband; yet, only one (Rasheed) believes in his right to physically terrorize his wives to the ultimate end (death), if necessary. True, Rasheed crudely forces himself on Mariam and Luke does the same with Meggie (both men make the first act of consummation extremely painful for their young brides), but Rasheed carries his physical assaults beyond the bedroom.
In The Thorn Birds, Luke is ecstatic to be married to the wealthy and beautiful Meggie Cleary. From his youth, Luke has been an indefatigable worker, shearing more than two hundred sheep a day, six days a week. The author portrays Luke as a man who worships industry and profit margins above everything else. Although he's attracted to his wife, he views her as a means to an end, both sexually and financially.
He initially makes his decision to woo Meggie on the assumption that her family might provide them with sizable land for her dowry. When he learns the Catholic Church is actually the true owner of Drogheda, he's disappointed but unbowed in his determination to snag a wealthy wife. Luke is the kind of man who loves "hard cash far more than what it might eventually buy him; not the possession of land, nor its inherent power, but the prospect of stockpiling rows of neat figures in his bankbook, in his name." Luke views Meggie as a woman who will help him realize his dreams.
Like Luke, Rasheed is wholly self-absorbed and egotistic. Luke looks at women as objects of pleasure; to him, a beautiful woman is only good for one thing. Beyond the confines of the bedroom, he has very little use for Meggie unless it concerns his pocketbook. Luke's desire for wealth far exceeds his desire for sexual satiation. Essentially, money is his god, and he makes no pretense of religious devotion. In the novel, Luke adamantly refuses to convert to Catholicism in order to marry Meggie; for her part, Meggie has to be content with being married by Father Thomas in the presbytery without the traditional nuptial Mass or blessings.
On the other hand, Rasheed proudly maintains all the outward appearances of religious devotion but secretly enjoys pornographic magazines in his spare time. To get to the heart of the matter, Rasheed's religiosity is only of a surface quality. He doesn't fast on most days during Ramadan, and his requirement that his wives wear the burqa is more a demonstration of his dominance over them than an indication of any sort of religious devotion.
The most reprehensible part of Rasheed's character is that he holds no reservations about striking his defenseless wives. When he discovers Laila has been meeting with Tariq, Rasheed becomes violently angry. He proceeds to use his belt to strike out at Laila; with great fury, he chases the terrified woman all over the house until he catches up with her and slams her against the wall. Cornered, Laila can do nothing but claw and scratch frantically at her husband. When Mariam tries to pry Rasheed away from Laila, he turns on Mariam.
In order to save Mariam, Laila slams a drinking glass against the side of Rasheed's head. Even though Rasheed is bloodied by Laila's defensive measure, he immediately turns on Laila and uses his bare hands to grab her by the throat. Mariam tries to pry Rasheed off, but she's unsuccessful. To her horror, she realizes Rasheed means to strangle Laila to death; if she doesn't intervene, Rasheed will kill both her and Laila. In the end, Mariam goes to the tool-shed and retrieves a shovel, which she uses to kill Rasheed. Even though it's an act of self-defense, Mariam is left emotionally scarred by the effort.
So, by all indications, Rasheed is more villainous, a reprehensible man willing to resort to murder to fulfill the dictates of his ego and perverted conscience.