your personal picture if it's a personal account OR an image of your website if you are promoting it.
I ran a test a few months back. I did 4 similar accounts with following the same people. The only difference was the profile pic. I assumed a pretty girls picture would get the most followers and clicks but turns out it came in second. A normal looking guys picture got the most. Third was a random logo and last was the account without any profile picture.
Some of the best profile pictures I've seen are ones that showcase skill and talent. For instance, most of the web designers or graphic designers would create a cool character for their profile picture. Companies often use their logos. If you are an individual however, it can be powerful to use a picture of yourself to create a sense of personal connection with your followers.
Use your own picture! And here's an obvious tip: SMILE! This does wonders. Well if you go to the dentist once in a while