This is the reason why I love Sherlock and John together. They are completely opposite but they bring out the best in eachother. Sherlock is always joking, but John is the serious one who keeps them grounded.
This has to be one of my favorite Moriarty faces ever. The other ones are his 'suprised' face, and most of the faces he makes at his trial. Not to mention every other face he does in the entire show, ever.