？
What do you think would be the best souvenir for her?
Literal
Her [toward-へ] [possessive-の] souvenir [target+topic-には] what [subject-が] most good [quotative-と] think?
Several layers at work: には stacks target に with topic は to foreground the souvenir question. お土産 uses the conventional honorific prefix お- (this word almost always appears with it). 何が一番いい is a standard superlative question pattern. The quotative と marks the quoted thought in ~と思う.