"Scent of Roses", probably one of if not the most tearjerking episode from the original series.
KITT's drowning scene in "Junkyard Dog". You could hear the fear in his voice as he's begging Michael for help, while he's sinking into an acid bath! Meanwhile all Michael can do is stand their helplessly while someone is trying to drag him away from the scene for his own safety.
Later, when they drag KITT out of the muck and Michael takes a look inside... only to find KITT eaten from the inside out. He then tries asking KITT if he's alright, only to recive no response.
The scene just before KITT gets blown-up in "Knight of the Drones". Michael and KITT are being targeted by a missile the're both aware will hit KITT no matter what they try to do, so KITT ejects Michael to safety. It's not so much that KITT did it, it's the fact that he did it without asking or insisting Michael push the button himself (most likely knowning full well that Michael would never abandon him). Not to mention what he says to Michael, no that really brings on the water-works.
KITT: Sorry pal...
Michael: What? (suddenly gets ejected from the car)