You made me curious about theses assessments. I copied the two directly related to V.Nabokov: http://www.theatlantic.com/magazine/archive/2014/04/who-is-the-greatest-fictional-character-of-all-time/358649/

Billy Collins, poet - Dilettante and monster, pursuer and pursued, seducer and seduced, Nabokov’sHumbert Humbert evades simple moral verdicts almost as well as he did 50 years ago. Lolita still works to implicate its readers in a reprehensible crime by addressing us as both witnesses and jurors and by encasing Humbert’s sins in Nabokov’s exquisitely playful prose.

Errol Morris, filmmaker and author -  Charles Kinbote—his enthusiasm for poetry; Bartleby—his engagement with others; Cain—his love of his fellow humans, even at a time when there were many fewer of them; Ahab—his passion for seafood; Meursault—his warm feelings toward his mother; Gregor Samsa—his unshakable self-esteem.

And I think VN would have appreciated the distinction between fictional creation and fictional character by M.Cunningham who answered the question as seriously as Billy Collins did without mocking the kind of “enquête” as the one promoted by The Atlantic:  

Michael Cunningham, author: Emma Bovary may not be the greatest of fictional characters, but she’s the greatest fictional creation. She’s selfish, frivolous, and dim-witted. She’s unfaithful. She’s vain. But Flaubert insisted so ardently on her right to our attention that he created a tragic, immortal literary figure out of a twit. That’s greatness.