Let me start this off
I was replaying Skyrim and was on my way to solitude to join the imperials. As I walked to the watchtower (the one with the first dragon fight) I see a dragon spawn in on the ground. As I go to get a closer look, I see a giant on its wings. I run up to fight the dragon, and it begins to fly away, with the giant on its back. As you would expect, the giant fell off and died. This was the only time I saw a dragon defeat a giant.