The Samsung Galaxy S8 has a very bright screen. I've read that on sammobile website they measured the maximum brightness of the S8 of around 44.70 cd/m2 (nits) and it can go up to 693.21 cd/m2 when set to auto brightness, where the phone sensor automatically detects the ambient light intensity and adjust the brightness of the screen for best visibility and also to save battery life. The s8 actually offers a screen brightness more than most of most current smartphones. Just don't expect it to be as visible as viewing the screen indoors. I found a nice video on YouTube by Gupta Information Systems that shows the Samsung Galaxy S8 screen's outdoor visibility in a bright daylight.
You can see that the visibility is good. It's better when viewing from the front than from the sides, but when viewing on front, you can still see things quite clearly and operate the phone's UI. The guy who tested the S8 used the phone in the default settings, which is set to automatically adjust the brightness based on the light intensity reading of the surrounding ambient light.
I have many Android smartphones at home, and all of them give me very terrible outdoor visibility. I just can't see anything when I use them in a sunny day outside. It just look like dark grey and even if I put my hand to block the sun it still very hard to read text and to use the UI. A very high brightness will obviously consume a lot of battery power and it's recommended not to use the phone with its highest brightness, especially indoors, which can be maybe too bright and not good to the eyes.
By the way, I went to the DisplayMate S8 test page to see what their brightness number show. According to DisplayMate S8 screen test, the Galaxy S8 has 5% to 19% (in auto brightness) higher screen brightness compared to the S7 and they measured a peak brightness of over 1,000 nits, which is very impressive. According to DisplayMate, this is a record high measurement! Even the viewing angle has been improved, so you an even better visibility when viewing the phone from the sides. In their article they also mentioned that what they called the "screen reflectance" also plays a significant role in the outdoor visibility, which in the S8 is better than the S7 and one of the lowest tested by this website.
If you want the best outdoor visibility, you should set the phone to auto-brightness, then it can go up to over 1000 nits. If you use to adjust it manually, just so you know that the maximum brightness is 610 nits, which by itself is quite impressive.
I wonder if in the future we are going to see a technology, not like Amazon's kindle of course, that can make a display like the S8 one be as visible outdoors as it is indoors, this would be amazing. I'm not just talking about bumping up the brightness to an ever higher value, but a technology that can maintain better visibility with the current brightness.
Bottom line, excellent brightness and very good outdoor visibility, most probably better than any other phone out there at the time of writing.