The accent wall is a focal point for the room. Paint the wall that you think this should be. This could be the wall that is seen when first walking into a room, it has the most furniture against it or maybe a particular piece of fine furniture or artwork.
The biggest thing is this wall and color should go with the rest of the room and its furnishings. I wrote a couple of articles about this; <a href="https://www.house-painting-info.com/choosing-an-accent-paint-color.html">Choosing an Accent Paint Color-Tips and Tricks</a> and <a href="https://www.house-painting-info.com/using-an-accent-paint-color.html">Using An Accent Paint Color-Create a Focal Point With an Accent Color</a>