At least with the insider preview build 10547, the embedded Photos app has now also a Slide Show (F5 – second icon in the top right menu) feature: so no need anymore to necessarily use other ways … that any way remain available. The only limitation I found in this app is that you are not able to easily import files from your phone to the PC, because it sees that pictures already on the Cloud (saved directly by the phone) and so it shows them but it does not save on the PC: I think that it should provide an easy way to do that because, very often, someone wants also to save locally his photos.
Anyway, if you still want to use the Windows Photo Viewer desktop application and you want to have your portrait photos be displayed properly (and not zoomed) remind to set to 100% (and not at 125% or 150%, as suggested and set as default) the Settings -> System – Change the size of text … : you will be asked for a reboot and then the slide show in Windows Photo Viewer will work properly also for portrait photos! So, waiting for a new release from Microsoft, the previous trick can solve the problem of showing portrait photos.
Another way to make a slide show is to directly use the File explorer feature: on the top menu, choose Manage (available only if you are in a picture’s folder) and then Slide Show: the problem with doing it this way is that you can’t begin the slide show on a specific photo (even if you click on a specific photo and choose “Slide show“, it starts from the first one – e.g. in alphabetic order, if so displayed).