What is really the best choice of a smartphone ?

I got a OnePlus 6 recently. OnePlus is a company that hasn't been around for too long so they're not as well-established as the likes of Apple/Samsung, but I found the OnePlus 6 to be really good value for money. For $529 you get a phone with a Snapdragon 845 and 6GB of RAM. This is the level of specs you would expect from the highest-end phones of the era that cost a lot more (for instance, the Samsung Galaxy S9 also has a Snapdragon 845 but only 4GB of RAM despite costing over $100 more).

The phone's screen is very big at 6.28 inches. Some might prefer the large screen but others might find it too big. Also, unlike Samsung with their Gear VR and Google with their Daydream VR, the OnePlus phones don't really have any good options for VR if that's something that interests you. Other than that, the camera isn't quite as great as the best phones but it's still perfectly good for the vast majority of people. Overall I would recommend it.