It depends on your preferences and what you intend to do with it. I'd certainly handle all 3 to check what you are comfortable with before buying.
The Pixel is the oldest, doesn't have SD expansion, but will have the best software support (updates will be faster and more frequent). I'd expect the Pixel to be smoothest in operation despite having the least powerful hardware.
The S8 is the narrowest, has the largest fraction of the phone taken up with screen, and has every feature (and every gimmick) Samsung are currently offering built in. Be aware that the internals are different in the US, where it's based on Qualcomm's Snapdragon 835 rather than Samsung's own Exynos SoC, so many reviews are of a different model - the differences should not be large, but there will be differences (some pros for one, some for the other).
The Sony is the largest, has pretty similar specs to a US S8 but lighter software than the Samsung, more conventional screen format, and a higher-MP camera (which can be a pro or a con depending on how you use it).
There are pros and cons to all of them, so it really depends on what features/aspects of a phone matter most to you.