Goodnight, Seattle: Part 2 - S11-E24
Trivia: In the pilot episode, 'The Good Son', there is a fight brewing between Martin and Frasier because Frasier wants to hear Martin actually say the words, "Thank you" to him, for accepting his father into his apartment to care for him. In the series finale, 'Goodnight, Seattle (pt 2)', when Martin hugs Frasier, Martin says, "Thank you, Frasier," to his good son.