Well, one reason this question may be causing you trouble is that it is slippery. The greatest declarations of love come in Act I, and the rest of the play is dedicated in large part to showing how much Regan and Goneril don't mean what they say. So, what you're really looking for is what Cordelia says, and how she loves her father more than her sisters.
She says, in Act I, scene 1, that she loves her father because he begot her (made her mother pregnant with her), breed her (caused her to be born), and loved her, and that she loves him only as much as is appropriate to their bond as father and daughter. Because she won't flatter him, her father thinks her sisters love him more.