Since Riven scales with AD and has no mana, the 2 core stats for her are AD & CDR.

The first priority for every Riven build should be maximal CDR (40%) and as much AD as possible.

Armor, HP, Magic Resist and Armor Penetration are situational.

The strenght of AD Casters & Bruisers lies in their ability to adapt to the enemy team's stats, because you can always sacrifice some AD in order to increase your armor and/or magic resist.

Situational items (3v3):
Maw of Malmortius is a situational item for riven that should only be used if the enemy team has an AP carry

Randuins Omen is also a situational item that is useful against AD carries.

(If the enemy team has an AD carry & an AP carry, you should replace hydra & frozen mallet with Maw of Malmortius and Randuins Omen)

Ionian Boots, Black Cleaver and Sanguine Blade/Bloodthirster are the absolutely necessary core items for Riven. The last 3 Items are situational and should be used either to increase AD, Armor Pen, Armor or Magic Resist.

It is extremely important though that the main focus of every Riven build is on attack damage, because all of her abilities scale with AD.

In order to kick *** like a boss, RIVENS ATTACK DAMAGE HAS TO BE ABOVE 350