The Galaxy S8 and S8+ will be capable of shooting and playing 4K video at 120 frames per second via an external monitor. But make sure that the 4K video should be included in Galaxy S8 playable 4K file formats such as 4K MP4, 4K AVI, 4K WMV, 4K MKV, you will fail to open some 4K files such as 4K MXF, 4K Prores, 4K XAVC/XAVC S on Galaxy S8.
The best phone amongst these phones would be the LG G7 ThinQ as it gives better specifications at a lower price.
Processor: The LG G7 ThinQ comes with the Qualcomm Snapdragon 845 processor which is better than the processors of the other two.
Camera: The LG G7 ThinQ has a camera of 16 MP + 16 MP with F1.6 aperture.
Check Full Device Specifications LG G7 vs LG V30 vs Samsung Galaxy S8
Galaxy S8 vs OnePlus 5
Both the phones have their pros and cons, I think you should go for OnePlus 5. OnePlus 5 has the following advantages over Galaxy S8 -
But it's not that Galaxy S8 does not have any advantages over OnePlus 5:-
There are many things which are common between them one of them is their processor's performance.
Verdict:- OnePlus 5 has many advantages over Galaxy S8 such as more RAM, more storage, Dual camera and the best is you are getting all these features and specs at a very good and relatively lesser price than Galaxy S8.
You can check the complete comparison on 91mobiles link:-OnePlus 5 vs Galaxy S8