My personal favorite would be What If...? (Vol.2) #26, because it was written by Kurt Busiek.
A What If...? story I would have liked to see expanded upon would be What If...? (Vol.2) #7
spoilers:end of spoilers
(it would have been interesting to see how much more good in the world Director Logan could have accomplished in this AU reality).
Definitely Mr. & Mrs. Williams (Simon & Wanda) from What If...? (Vol.2) #5
...but with a happy ending for the married couple, if possible.