Some prefer scailing MR, but I think runes help more especially in early games. Get MR runes unless the enemy team got 4 AD champions. You need MR early on anyway because the enemy mid laner will be roaming a lot if you are winning.

I think my 21 Offense tree is the most efficient one for Riven. Riven doesn't need attack speed or critical rate because she heavily depends on burst damage from QQQ and R, not sustained damage like ADC champions do.

Change the Defense tree according to your lane opponent. Get 3 MR mastery instead of armor if the top is an AP champion.

Item build is the main reason I decided to write this guide. I don't think I can much explain other things because you need to get experienced by playing Riven a lot. But you'll not know much about item builds if you believe a myth like "I have to get Blood Thirster first" or "I need boots first."

Riven's Q increases her AD ratio a lot, so adding even a little bit of AD will help her. This means, don't wait until you get over 1600 golds for B.F.Sword, a ward, and health potions. You won't get a chance to get a free farm like that unless your enemy team is full of newbies.

You got your Q and E. Why would you buy boots first when the enemy can't run away from your combo with speed yet? I'd rather kill more quickly, because I don't want to chase the enemy to the end of the map.

I personally believe Ravenous Hydra and Black Cleaver are the must for Riven whatsoever, because the prerequisits for them aren't too expensive. You can constantly buy AD items so that you won't have damage loss gaps.

If you have enough golds to buy Elixir of Fortitude and another item but you got only 1 spot left, buy EoF first and use it before you buy another item. It might cost a lot in total, but it's worth! Your goal is to win the game, not to be economically efficient.

Here is my recommendation for a successful lane strategy:
After killing the opponent or finding that your opponent recalled to base, push the lane and press B. When your opponent returns to base, he will come back with more itmes and more likely you'll have a hard time. Don't think that you will win again when the enemy comes back with heavier items.

These are just examples of what you can do. Think what your spells do and react to each situation.

When it is a 1 on 1 situation, use R when the enemy's HP is low enough so that you can kill with one combo with R on.

During early lane period:

vs. hard AD like Darius: Q - W - Q - auto - Q(stun) - auto - E to get out of range
vs. Riven's foods: Q - auto - Q - auto - E - Q(stun) - auto - W - auto

In teamfights:

R - E - Q - auto - W - Q - auto - Q(stun) - auto
Target the enemy ADC and AP first, and use R again whenever the target is trying to run.

Don't forget this.

Riven will be OP only if you are a good team player. It doesn't matter if you win your lane because you can't win alone against 5 enemies. Your enemy team will chase you down until you rage quit if you refuse to cooperate with your team and play like in a single mode.

Don't cry until the end if your team steals your kill. It's a team game. Don't forget to continuously compliment on your team whatsoever. Don't blame on others, and try to make everyone hopeful when your team is losing. Comebacks always happen with a full effort of teamwork.

You will win more if you can do this. Vice versa, you'll lose more with a negative attitude even if you are a skilled player. I've learned this in a hard way.