In my honest opinion I would have to say that whichever of those 10 spoilers are the least well known would be the biggest spoiler today.
Personally I know of the Shiek spoiler only from playing the Super Smash Bros series. I also know the Princess, Samus, Rival and Wesker spoilers from playing their games too. However I have never played Portal 1 but the cake spoiler became a very well known meme so I know about the cake.
The other 4 spoilers would be spoilers to me due to me not playing their games and not them not being as popular a meme as the cake. --- Looks like they have spilled coffee on the server again.