This question can be resolved fairly easily when you have a clear grasp of the themes as they relate to the message Balzac's story conveys. Two very important themes are (1) the harm done by over-indulgent parents and (2) the corruption that extends from the highest social classes to the lowest: "Succeed! . . . succeed at all costs" (Vautrin).

The character Père Goriot (Father Goriot), the titular character, is the primary representative parental figure and Rastignac is the primary representative child figure ("You’re a good little lad"). Goriot's role as Father Goriot is of primary importance since the primary relationships center around him: his two daughters, Rastignac, and the attention of the merciless boarders. Thus he is the primary representative of the theme of parental harm through over-indulgence.

Since the story is an examination of parental harm and social corruption, and since Balzac symbolically links parental harm with social corruption, and since this link results in a representation of parental harm as parental corruption, and since Goriot therefore represents both major themes, it makes sense that Père Goriot would be the character who embodies the essence of the story and, therefore, is the titular character: "The finest nature, the best soul on earth would have succumbed to the corruption of such weakness on a father’s part."