If you want to make a best gaming PC, build it yourself!
You'll HAVE to get Windows 10 since it require Direct X 12 for more games, however note that if you want to play classic PC games relying on 9.0c or previous technology, you'll have to get 8.1 as DX9 is gone from 10, sadly.
8GB of RAM minimum, 16GB recommended (for games which are unoptimised)
750GB or more disc space depends on how many games you intend to play but most games need around 25GB or more if directly installing them
HDMI cable and a HD TV (computer screen won't be always great for games :D)
i7 Processor (or something with over 3GhZ; but not AMD because that processor is bad for games)
SuperFast Internet if you want to play online
NVIDIA graphics card or better, just make sure you look for a good one depending on the games you play and stuff.
[DEL]An account on my website[/DEL] (joking)
Steam Account to buy games
Oh, and don't forget the saying: "Gameplay and the fun level make a game shine, not fancy graphics or audio" - DixieKongJD
I'm not too sure on the money you will need to spend but it could possibly come to over £600 (roughly $850) if you build it yourself but the results will be satisfying for you.
I hope this helps. @Yusheec on Twitter might also be able to help you with this, too.