You can always try and figure the best builds out for yourself but under the Armory menu there is an option to customise characters with the 'Pro Build'. This feature enables you to find some of the best builds for each character and equip them as your default build. When you equip a 'Pro Build' the recommended items that pop up will match those that the best players use. These may not work all the time as they are based on popularity and not win rates, but nevertheless they will be an impovement over the default builds the game recommends.