After studying over 200 TED talks, I have found that the best backgrounds are the ones that are the simplest. Using a simple background keeps your text readable. Aim for a solid color for the background because this allows you to create a clean slide.
A clean slide allows your audience to focus on the text as opposed to being distracted by a busy background. You may, however, use a slight gradient fade for your background to make your background more appealing but only if the gradient is subtle and not too distracting.
In the sample slide below by Pankaj Ghemawat, who presented at the TED Global 2012, Pankaj uses a simple black background:
The reason Pankaj’s slide works well is because he has a clean black background that allows the audience to focus on the statistic that he is presenting. In addition, the text is white. This creates a great contrast that makes the text easily readable.
When creating your presentation, choose a simple background that contrasts well with the text.