A lot of stuff involving the character of Ralph, but the best moment involving him would have to be during "Cheerleader Casey", where puts on an In a World/SUNDAY! SUNDAY! SUNDAY!-style voiceover voice. skip to 9:09 to hear it, and then to see the other actors reactions in a blooper real, skip to 2:31.
In Vacation With Derek, Derek's line "GIMME THOSE KEYS!!!". Not a funny like by itself, but the delivery is priceless.
Olga, the Russian Driving instructor in "Driving Lessons", particularly Casey's reaction to finding out she actually exists, despite thinking blowing Derek off about it before.