Aspect ratio is an extremely important but unfortunately often overlooked feature. In our opinion, the best aspect ratio is 4:3. This suits a mix of both video and photo playback while offering the most versatility and suitability to photos sent from a mix of different sources. Most of our smartphones shoot in 4:3, whereas many DLSRs capture 3:2. You’ll get a far better match displaying a 3:2 picture on a 4:3 screen than you would by displaying it on a 16:9 display.
You’ll find that with 16:9 (most common type) and 16:10 displays, most of your pictures won’t fill up the whole screen. They’ll often be surrounded by black boxes or will have to be auto-cropped/stretched to fit the display.