I'd say there are some good examples of real love in this book. For example, Joe and Biddy have an innocent and happy relationship. Wemmick loves the Aged parent. Are there good examples of passionate romantic love? Not really. The love is more familial. The message seems to be that romantic love is dangerous. Pip's passion for Estella results in only heartache. Miss Havisham is ruined by love, and teaches Estella to be so cold that she can never have a normal relationship with anyone.
In addition to Joe's love for Pip, Magwitch shows the same sort of fatherly love toward the novel's protagonist. He is, also, the character who is ultimately able to teach Pip how to love unconditionally. Through years of hard work, Magwitch repays Pip for providing him with food when Pip was a child. When Magwitch returns to reveal himself as Pip's benefactor, Pip is utterly repulsed. However, after spending time with Magwitch, Pip understands and appreciates what Magwitch has done for him. It is Pip's interaction with (and care for) Magwitch that helps him to understand the love that others, such as Joe, have shown him throughout the course of the novel.
Friendship is, indeed, the greatest love, as proven by Joe Gargery. For, friendship imposes no conditions upon the other person. With Mrs. Joe, Joe's love for her makes the imposition to him that he must consider how hard his own mother worked and he feels he must make allowances for his wife in order to spare her such a harsh life.
However, with Pip, no allowances, no conditions must be made. His refrain throughout the novel says "I love you, I forgive you, I want to be around you":
"Ever the best of friends, aren't we, Pip, old chap?"
With a question like this you are going to find that people's views with vary depending on their own reading of the novel, so remember that there will be no "one" answer to this question, and as long as you can back up what you say with reference to the text that is fine.
For me, however, the best example of love in Great Expectations is the love shown to Pip by Joe. Another key theme in the novel is that of parents and children, and how the novel is full of substitute parents for various characters, and Joe, although he is only related to Pip through marriage, really assumes the role of father in the text. Although at the beginning of the novel, he is shown as rather a weak character, being unable to stand up to Mrs. Joe and protect Pip in the way he needed to, and only being able to "pour more gravy" to console him as he is being insulted over the Christmas lunch, as the novel develops he shows that his love for Pip is unchanged and constant. It doesn't matter how badly Pip treats him, his love for Pip remains steady in the face of Pip's rudeness and snobbery. This is clearly shown at the end of the novel when it is Joe that comes to tend Pip back to health after his fever and pays off all of Pip's debts, in spite of how Pip has treated Joe. Throughout the novel Joe displays a pure love that is not based on self gain. We see another example of this when he refuses any sort of compensation from Jaggers for taking Pip away from him.
As Joe himself says, at Pip's surprise that he is there looking after him:
"Which, dear old Pip, old chap," said Joe, "you and me was ever friends. And when you're well enought to go out for a ride - what larks!"
To Joe, nothing has changed - although other characters in the novel grow in their love, Joe's love has been the same throughout, as he shows through his self-sacrifice and care of Pip.