Here are my top 10 ranking for the story "What if" one-shots.
1) What If? Annihilation
2) What If? Civil Wars
3) What If? Astonishing X-Men
4) What If? Secret Wars
5) What If? Thor
6) What If? House of M
7) What If? Fallen Son
8) What If? Secret Invasion
9) What If? World War Hulk
10) "What If... The Amazing Spider-Man Had Not Married Mary Jane?"
What about your own?