It doesn't seem to be available on YouTube, so here goes:
I can't remember whether "Homer needs Heart Surgery" is the actual title or just the premise of the episode, but anyway that's the plot summed up for you.
My favourite scene starts with Homer just before the operation, as he is visited by Bart and Lisa. Homer knows that he might not survive the procedure, so he wants to say something poignant and inspirational to each of his kids in case he never gets another chance to do so. Being Homer, however, he's unable to think of anything, so Lisa leans over and whispers a suggestion in his ear. Homer thanks her, and delivers a genuinely touching farewell-message to Bart.
Of course, Homer can't think of anything worthwhile to say to Lisa either, so the logical conclusion has to be that he recieves a suggestion from Bart for this purpose.
Yeah.
Bart whispers something in Homer's ear, whereupon Homer says to Lisa:
"I guess it's time to tell you...
...you're adopted, and I don't like you!"
Of course it's fairly obvious to Lisa that it's just Bart being a bozo as always, but it still takes her a brief moment to realize this, and the shocked expression on her face is absolutely priceless.
(To Bart's credit, he does follow up by having Homer deliver a more heartfelt message.)