Watch Caps are very popular. I most commonly see a simple watch cap that is basically 2x2 ribbing throughout. Here are three Watch Cap patterns that I have knitted.
This is a classic pattern that has many different names on Ravelry.
This was a nice pattern with some variation to keep it interesting. It is Classic World War II Watch Cap.
And finally, A Hat Fit For A Boyfriend is another frequently knit hat. It is a classic that looks great and fits well.
My goal is 100 hats from 100 different patterns and I have completed 17. I am ready to cast on the next one and am looking for a pattern. I'm off to the Hats for Sailors Finished Projects page on Ravelry to see what others have been knitting.