The chipmunks and Dave placing the collar of their dead kitten on a plaque. It reads "In loving memory of Cookie Chomper III".
The end of that episode would qualify as well.
The chipmunks being reunited with their real mother.
"The Wall", the Chipmunks sing a song called 'Let The Wall Come Down' and through the Power of Rock and Power of Love bring the Berlin Wall down.
Fridge Brilliance: This aired the wall actually came down. Alvin said "it was all just a dream, but it doesn't have to be." Eleven months later...dream became a reality.
The end of the episode where Dave dreams about what The Chipmunks would be like without him around. It ends with a wonderful song that shows just how grateful they are to have him. Could also qualify as a tearjerker.