Firstly, please be careful about assuming how any reader feels about a character in a literary work. Responses to literature are varied and complex. That said, I will try to provide an assessment based on your question.
Let's start with Tom Buchanan who, one can generally assume, is the most loathsome character in the novel. He is racist, pompous, a philanderer, a snob, and someone who thinks that his money entitles him to treat people disrespectfully. In some ways he operates as a foil for Gatsby. Buchanan comes from money; Gatsby is a "self-made man." Gatsby epitomizes a romantic ideal; Buchanan is the man whom Daisy married out of practicality, as a response to the expectations of a woman of her social standing.
In discussions in which I have participated, readers generally tend to be more sympathetic to Gatsby's faults. He is a man who is holding on to the past and to his love for a woman who is incapable, it seems, of loving anyone. He is also a man who has made the fatal mistake of thinking that, by becoming rich (the ultimate measure of success in American society), people will forget his impoverished upbringing and he will be welcomed into the upper echelons of society. This does not turn out to be true, as both Daisy and Tom remind him that he is of a lower status than they.
There is considerably less focus on Nick Carraway's faults. He is the narrator of the tale. In this novel, his function is that of an observer, so he does not spend much time thinking about his own personality. If the reader is meant to identify with anyone in the novel, it is him. He is the most democratically-minded one of all of them. This is especially true in regard to his final assessment of Tom and Daisy Buchanan:
"They were careless people...they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made..." (179).