Among the celebrated Boom writers (García Márquez, Vargas Llosa, Fuentes, and Cortázar), Cortázar is apparently the one he most admired. As for Carlos Fuentes, he also had some things to say. He thought that Octavio Paz is "more universal" than Fuentes, that Paz is a "more interesting" writer of prose (in his essays) than Fuentes, and if he had to sit between them, he'd rather "sit closer to Octavio Paz than to Fuentes."
Bolaño's writing is seen as a break from the magical realist mode of the Boom writers and their imitators. His fiction is a sort of reaction to the previous generation's realism. However, it doesn't mean that he entirely objected to the literary outputs of the rest of the Boom writers. Possibly, he didn't like their politics or he just didn't like them as persons. But still he was an avid reader of their books. In a 1999 interview with two writers of a Chilean magazine (one of his published interviews in English), he shared his positive assessment of the (now) two Nobel winners.
Soto & Bravo: Perhaps the emblematic figures of the [Boom] movement were too adored, an injustice for quieter figures like Monterroso and Onetti, who are vindicated more and more. They’ve stayed relevant with the passage of time.
Bolaño: I don’t believe so. The literature of Vargas Llosa or García Márquez is gigantic.
Soto & Bravo: A cathedral.
Bolaño: More than a cathedral. I do not think time will harm them. The work of Vargas Llosa, for example, is immense. It has thousands of entry points and thousands of exit points. So does the literature of García Márquez. They’re both public figures. They’re not just literary figures. Vargas Llosa was a candidate for president. García Márquez is a political heavyweight and very influential in Latin America. This distorts things a bit, but it shouldn’t make us lose sight of the position they have in the hierarchy. They are superiors, superior to the people who came after and, to be sure, to the writers of my generation….
I suppose, though I’m not entirely sure, he didn’t answer in an ironic tone.