1. Stephen Hawking
The last thing you want to happen when you're presenting the one and only Stephen Hawking with your paper is to find there's an arithmetic mistake. Certainly, you don't want to make matters worse by fainting. I guess this wasn't how Sheldon Cooper imagined meeting one of his heroes. In 'The Hawking Excitation,' Howard takes up the role as Hawking's wheelchair engineer when the Scientist is visiting California. Obviously, once Sheldon hears about he wants to meet him. Howard has a bit of fun with this, making him wear a French maid costume, take Mrs Wolowitz shopping and forcing the super-genius to pay him a compliment. It's certainly a iconic moment for the show, and the culmination of a lot of effort. They'd tried to Hawking on the show previously but he was too ill to take part, so to actually see him here was a great touch.
What is you favourite Big Bang Theory cameo? Let us know in the comments below.