There are several things to consider when purchasing a PSU, but here are the most important:
- Does it have enough wattage to support my components and any upgrades I might want in the future?
- Is it made by a reliable company and is it made with quality components?
- What is its power efficiency? (How effectively it converts the power from your outlet)
- Does it have enough cables or sockets to support any future upgrades I might want?
There are obviously aesthetic requirements as well, like whether it's modular, whether the color will match your build, and sometimes special lighting.
That said, picking a PSU is one of the easiest components to choose. Here's the one I went with on my recent build, with a 6700k and a 1070: EVGA SuperNOVA 750 B1 80+ BRONZE, 750W. This unit has more wattage than I currently need but it leaves room for upgrades in the future.
EVGA is a reputable brand, this unit has great reviews, and it's relatively cheap. My only complaint with it is the mesh surrounding the cables don't completely block the colors of the cables inside, but that's not noticeable once it's in the case.
If money isn't a concern, go with something fully modular with lots of plugs, and higher power efficiency, like this: EVGA SuperNOVA 750 G2 80+ GOLD.