Pip and Estella from Charles Dicken's Great Expectations. Pip's unrequited love for Estella, a young woman trained by her wealthy, adoptive mother to treat men cruelly because of her own broken heart, is beyond Pip's reach, both socially and emotionally. Her beauty compels Pip to make himself a better man in the hopes that she will fall in love with him once he's a proper gentleman, even though she treats him badly. The complex nature of their relationship is one of the most interesting pairings in fiction.
I will also add Edmond Dante's and Mercedes in Alexander Dumas' Count of Monte Cristo as an interesting fictional couple. The strength of their love for each other even after several decades of separation including the time when she marries another man and has a son with him is inspiring.