Stone Cold at Wrestlemania 17.
It was something that at least I did not see coming at all, so at the time it came as a total shock and I couldn't believe my eyes when he shook hands with Vince McMahon. The same man that tormented him and made his life a living hell for years, and vice versa, I just couldn't believe it when they shook hands but I think Jim Ross said it best during that moment...
"Austin is shaking hands with the devil himself!"
Another good one was The Rock joining The Corporation at Survivor Series 1998, and Stephanie at Armageddon 1999 where she fully aligns herself with Triple H.