Today's movies and games are best enjoyed in a widescreen format at a 16:9 aspect ratio or above. In 4:3, those cinematic moments will look stunted with black strips along the top and bottom. There are a host of minute variations on each ratio, but at the end of the day choosing between these depends entirely on your personal preference.
And the very far-out option, if you have a little extra cash to blow, is ultra-wide aspect ratios like 21:9 and 32:9 and their variants. These will provide a much more immersive, encompassing experience. Or literally, encompass yourself with a curved monitor, up to you.