Hi
Here's the thing. It's all around old content set bonuses. There are certain ones which will probably give you the most damage, so I guess would be considered BIS, but generally it's about basing yourself to a certain playstyle. There are some set bonuses which will be pure damage, and some which will be about utility, both can work just as well. Have a look at all of them and choose what sounds the best to you and your particular playstyle.
The general gearing process is:
2x 1 set
2x 1 set
pvp gloves (for bonus if useful)
2x t6 as bracers/belt if the set bonus is useful.
AB boots (move speed) (hunters/enhances have access to the Earthen Netherscale Boots which are better than AB boots)
Legendary chain ring
Upgraded quest ring or Revered ICC ring (exalted gives a gem slot but loses more stats from scaling)
Crafted weapon/offhand
Crafted Trinket
Legendary Cape or Crafted Cape.
Crafted offpiece item if you aren't already capped. Note: the crafted offhand items do not count towards your 3/3 crafted limit. This DOES NOT apply to crafted shields, they count towards 3/3. Crafted off-hand weapons for dual wielders do count towards 3/3.
Anything else is quest upgrades or quest items, usually.
The other important thing to note, is the randomness of the offpieces. You will be building your character around the upgrades you get in your Spire/Nagrand questing/rares/treasures. It's good to have a general idea of what you want to gear, but nothing will be certain until you've done those zones completely. You might get a 590 ilvl thing and be meh, or you might get an upgrade to 620-630 and suddenly it's BIS.
Make sure to build a bunker/warmill asap, as it doubles your chance for item upgrades, which applies to quest items, rare drops, and treasures.
