I have to go with Doug Emhoff. He was warm, funny, natural, human, and obviously in love with his wife. Making the pitch that Kamala is compassionate and tough was very effective. He was completely and relaxingly authentic in a likeable and sometimes self-deprecating way. Thoroughly enjoyable speech.
Honorable mention goes to Michele Obama. She absolutely killed it with her takedowns of Trump. Clearly she is not over all the bullshit the orange shitstain has said about her family. Good job!