Chest
(Health/Skill Haste/Enemy Armor Dmg/Health on kill)
+ Ammo capacity
Mask
(Enemy Armor Dmg/Skill Power/Health on kill)
+ Dmg to Elites
Knee pads
(Health/Skill Power)
+ Dmg to Elites
+ 2 fav resistances
Backpack
(Health/Skill Power)
+ (Ammo capacity/fav resistance)
Gloves
Prim Weapon Dmg
+ Enemy Armor Dmg
+ (Sec Weapon Dmg/Skill Haste/Health on Kill)
Holster
(Skill Haste/Health)
Alternatives in parentheses should be selected according to each specific build: solo, damage, skill, etc.
Source: 1.8 Guide.