The Wilhelm is great! My dad and I imitate it all the time. One time we were moving stuff in the garage and my dad lost his balance on the step ladder. He only fell one step or so, but after he regained his balance on the floor he chuckled and did the Wilhelm, and we both cracked up.
I think my dad once said he cracked up when he saw
Star Wars in the theater and heard the Wilhelm (though he didn't know the name at the time) because he remembered it from some older westerns when he was a kid and hadn't heard it in years.
I love Ben Burtt's cameo in
Return of the Jedi where he does his own Wilhelm imitation when Han knocks him over the railing.