How to know which gear is the best?????

The best? You do a lot of math, preferably using some kind of script and simulator that will help you test thousands of combinations per second How to pick decent gear? You pick a set that matches what the character does, then you pick 3 pieces from the set that best match the attributes of the character you want (matching the character's main stat is a really strong boost), and have passive boosts that match the kit of the character. Then you pick the last piece from any set that just has the best raw stats that you want. How to get good gear without thinking? Go to a website with guides and builds and just copy what they say.