One of the biggest things you need to decide when choosing a smartphone is which operating system to use. Although there are others around, it's best to stick with the big names including iOS, Android, Windows Phone and potentially BlackBerry. There are no Windows Phones in the chart at the moment but this might change with the Lumia 950.   See:  Best Windows Phones 2015 . If you're already using one then it might be best to stay in that camp – especially if you've invested time and money in its apps. However, it's not difficult to switch so you should consider them all. Note there aren't any BlackBerry devices on the market at the moment which we think are good enough to make this list but check out our  BlackBerry Classic review  and  BlackBerry Priv review . Design will come down to personal taste and most of the top smartphones now have a very thin and light chassis. The best smartphones typically use premium materials like glass, aluminium or even ste...