If you're willing to wait about a month or so, the new S8 will be released and should cause the S7 to drop in price. The S7 is a great phone, has one of the best cameras at the moment (Pixel beats it but that's when the settings are configured for the conditions, the S7 has the best out of the box auto camera) and overall it's just one of the best phones on the market at the moment.
If you don't want to wait then you could buy a OnePlus 3T like you said, since that's also a very good phone, has lots of android modding support and the specs are the best for the price.
Just depends on what you want to do really. I wouldn't recommend getting an S6 since it doesn't have an SD card slot and compared to the S7 it's not very good. Also, if you get a Samsung phone, don't get an edge version unless you have very good hands and know that you wont drop it, since the glass is pretty weak on the edges and will smash when dropped.