In canasta, What is the top card of the discard pile?
The top card of the discard pile is always the most recently discarded card; the card physically on top of the rest. The fact that it is a wild card in this case doesn't change anything. Discarding a wild, like discarding a black three, is a way to ensure that the discard pile cannot be taken no matter what this turn.

From the rules:


  Note that you can never take the discard pile if its top card is a wild card or a black three.


The rules do not really define "top" here, but it is using the common English language meaning, "the highest or uppermost point". If the newly discarded wild card were not considered the "top" card, then the quoted rule would make no sense, as there would never be a way to have a wild card be the top card of the discard pile.