I'd recommend you try them all, especially if you shoot still pictures. There is of course a reason we call vertical pictures "portrait" pictures and the horizontal ones "landscape" but if you have a digital camera play around. I have seen portrait pictures taken in square format and "landscape" pictures taken vertically. Dare to explore, the findings are rewarding and will help you sharpen your ability to evaluate pictures.
In all occasions try to do justice to your subject.
Keep the world smiling, take beautiful pictures.