I don't suggest buy an S6 at all. It's a decent phone but it marked the beginning of a new era of S-series devices from Samsung and it definitely shows. Quirky design (the S7 is just so much more polished) and awful battery life mean the S7 is a far, far better choice. I'd take an A7 over the S6 as well, but that's just me.
I would personally recommend the G6, but I am biased! The S7 is a good device but the G6 gets you a lot of features and is a way more economical bang for your buck; especially if you're getting it via your carrier. What is important to you in a phone? That would also influence the choice you'd make in a phone. Of course, with the S8 being out, you may be able to snag an S7 for a good deal, as well.
A7 is mid range phone ? I think, idk...
But hovewer S7 & G6 are flagships, but S7 is last years flagship.. So between these 3 i would definetly go on the G6.. Bigger screen, better battery life, newest, better camera, it will last longer.. And etc....