"Once in a Blue Moon," when Baby Smurf enters the village. He even gets the usually grouchy Grouchy Smurf to warm up to him, even to a point acting as Baby's guardian when the threat of Baby having to return from when he came arises. It allows Grouchy to show a softer side and develop some of his character.
That episode "The Dark-Ness Monster", where Brainy loses his glasses and has to rely on Clumsy to lead him around. Actually, Clumsy tends to instigate these by his mere presence.
The episode "The Smurfiest of Friends" shows just HOW much Clumsy likes Brainy as a friend - he's the only one who wants Brainy to attend the party thrown in his honor (and this is AFTER Brainy ended their friendship); he immediately goes looking for Brainy once the Smurf disappears; when he does find Brainy, he's perfectly willing to take his place as a prisoner if it means Brainy can get away, and as they both head back to the village, he tells Brainy that he would rather be with him than his other friends...excuse me, my heart just melted.