The opening discussion about why 73 is the best number was classic Sheldon, and I loved how he built such an elaborate case for something so ridiculous.
The superhero subplot was equally hilarious, especially Raj and Howard's gym showdown, where their attempts to settle who would be the hero and who would be the sidekick became funnier precisely because neither could get on with it.
Amy's unexpected attraction to Zack was a promising setup, and her symptoms gave Sheldon plenty of opportunities to be his wonderfully awkward self. However, I felt this storyline lost some steam as it went along and became a little repetitive. The highlight was the quieter Sheldon-Amy moment involving the brain dissection, which added an interesting touch to their unusual connection.
7.5/10: Overall, this was a very funny episode with two strong comic ideas, even if Amy's story didn't quite sustain its initial promise. Sheldon and the number 73 might still be my favourite part.