The Tan Sao...
The Tan Sao is the simplest move and also very effective during a fight. Simple and to the point, Doesn't need much training to master.
Comment from Rob: Great point. Tan sao is one of the few Wing Chun blocks/defensive positions that comes naturally to most people. It seems to work by using the natural human structure.
Many other blocks/defensive positions are physically unnatural, like the fook sao, bong sao, and lap sao, bill sao, etc.
Thanks for your input!