The five most popular ones are (by far):
- 19.2%: this
- 18.5%: function
- 18.2%: if
- 17.1%: return
- 12.6%: var
As an interesting aside, CoffeeScript eliminates two out of the first five completely (return and var), and has much shorter ways to express function (->) and this (@).
For more information, including a diagram of the distribution of the keywords, consult Ariya Hidayat’s article.