First of all, many jokes are repeated countless times through the series and yet they're funny every single time.
This Troper finds himself cracking up every single time he watches the "Posada" episode: Mr. Barriga is sweeping the patio's floor, unaware that El Chavo is nearby, swinging a little piñata around in expanding circles, approaching inch by inch to unsuspecting Mr. Barriga's head... and then suddenly striking Quico, who was behind of El Chavo, off-screen.
Quico and Ñoño ask Don Ramon about El Chavo.
Don Ramon: He was here a minute ago... looking for a fat guy and an idiot.
Quico: He was looking for us!! =D
Quico and El Chavo are giving a bath to the latter's new dog (Which is named "Little Ramon"). So, when Quico turns, El Chavo accidentally brushes soap into the boy's shorts. Cue to his response:
Quico: El Chavo... I think I made a mistake.
The episodes involving "El Festival de la Buena Vecindad", especially the cultural acts from the kids:
The Poem/Story of "El Perro arrepentido" by El Chavo. Complete with overly dramatic gestures. This is also the only poem the kid knows, so it appears every time.
"Soy virgencita / riego las flores / y lo que sigue / me se olvidó..." ("I'm a little virgin / I water the flowers / and what it follows / I just forgot")
The "Sapito" (Lil' Froggy) song that Quico sings. The song is a cutesy frog-themed counting song (in the style of the "100 bottles of beer in the wall" travel song), setting a Overly Long Gag that only ends when someone intervenes around the sixth or seventh frog, leading to the answer:
Quico: But if there is still about 100 froggies left! :(
Quico trying to recite the poem "Madre Querida" ("Dearest Mother"). We say trying because El Chavo is The Heckler to his performance, constatly interrupting the very corny poem with hilarious parodic rhymes.