This is my favorite joke of all time, but before I type it, part of the comedy effect is in the proper delivery:
A horse walks into a bar
The bartender looks at the horse and says
"So, why the long face?"
I've told this junk hundred, maybe even thousands of times; I've heard it even more than that. I know the delivery that works best and could do it in my sleep, or so I thought....until I choked, and somehow, the choke ended up being funny (for completely different reasons).
A group of us from the old Computer Software Plus gang had just had a great lunch together and we were hanging out on the corner for a bit as we were winding down. At some point, an elderly couple passed by and I spontaneously decided to stop them, the lady in particular, and do that joke.
The joke was on me though, because despite my experience with that joke, I totally flubbed it, missed part of it and had the timing and delivery totally off. All you could hear were crickets and combined with the look on this lady's face and her comment, "Really? You could do better than that!" in her thick Brooklyn-Jewish accent got the crowd, including me, laughing.
Here's a cool spin on this joke: https://www.youtube.com/watch?v=CUjVaaT60qY