HP Pong (1996)
You cannot write a "best of" list when it comes to online advertising and not mention this execution. Simply put, it's the one that started them all.
In a time when banner ads were a relatively new idea and the ones we were seeing were mostly static GIFs (with minimal colors) or maybe a more realistic JPG here and there, this bomb dropped and blew the doors off of everyone. Somehow, a then-small shop called RedSky had figured out how to recreate the classic video game Pong in a 728x90 banner ad, and it did it at an extremely small file size.
If you could have seen my face when I first saw this ad, it probably resembled something akin to Roger Rabbit's eye-popping, tongue-wagging surprise face. Followed by the question, "How the hell did they do that?" I promptly set about learning just that.