I decided to make this guide because I didn't see anyone build Riven the exact way I'm building her. I've had great success with this build so I wanted to share it so other's could benefit from it too. This is my first guide so constructive criticism is very much appreciated. Trolls, don't bother.
I will at first keep this guide short and sweet, outlining my item build, to get it out there and update it later with more detailed information/strategies.

Marks
My choice for marks is self-explanatory. The best marks are armor penetration and magic penetration. Riven does physical damage, so Greater Mark of Desolation is the clear choice.

Seals
For seals you can either choose Greater Seal of Vitality or Greater Seal of Armor. Flat armor seals add more survivability early game than the hp/level seals. I decided to use Greater Seal of Vitality because it adds to your damage after you build your Atma's Impaler.

Glyphs
I chose Greater Glyph of Scaling Cooldown Reduction because with them, my mastery build and Youmuu's Ghostblade you have close to maximum cooldown reduction even without the blue buff. ( Crest of the Ancient Golem) You can switch them out for Greater Glyph of Magic Resist, but if you do that I'd suggest you to switch Mercury's Treads for Ionian Boots of Lucidity (And switch some mastery points from the utility tree to the offensive tree because you'll be going over the max cooldown reduction if you take Intelligence .) because cooldown reduction is good for Riven. I feel that the tenacity from Mercury's Treads is essential to your survival with Riven so I prefer Mercury's Treads + Greater Glyph of Scaling Cooldown Reduction.

Quintessences
I'm using greater quintessence of desolation to do more damage. With The Brutalizer and later Youmuu's Ghostblade you'll be doing almost true damage to squishy champions.
If you want to have more survivability early-game you can use Greater Quintessence of Health

My mastery build looks a bit unusual. I wanted to get Sunder from the offensive tree and Intelligence from the utility tree, thus the 11/0/19 split.

With Sunder and your rune setup you'll have 31 armor penetration at level one. Intelligence is for the cooldown reduction.
Getting both Sorcery from the offensive tree and Intelligence gives 9% cooldown reduction from masteries alone.
Riven doesn't need Archaic Knowledge because none of her skills do magic damage.

or

SS

and

SS

Get Flash or Ghost and Exhaust.
I prefer Flash + Exhaust. If you get Ghost, switch the mastery Blink of an Eye in the utility tree for Haste in the same tree.

Core items:

Doran's Blade is the perfect starting item. Extra hp, damage and a minor amount of lifesteal.
On your first trip back buy Avarice Blade, Boots 2-3 Health Potion and a sight ward.
If you don't have enough money for both Avarice Blade and Boots buy whichever you have enough money for.
Avarice blade is bought early for the gold/5 seconds passive. Riven doesn't really need critical strike, but Avarice Blade will later be combined with The Brutalizer to form Youmuu's Ghostblade.

On your second trip back, buy Avarice Blade or Boots if you didn't have money for both of them on your first trip back.
Upgrade your Boots to Mercury's Treads.
If you have enough money buy The Brutalizer, 1-2 Health Potion and 1 sight ward


After you have built Mercury's Treads follow the item purchase order until you have finished Atma's Impaler. (Your 5th item, not counting Doran's Blade.)

When you have finished building Atma's Impaler it's time to choose your sixth item. Sell your Doran's Blade and decide whether you need more damage or survivability.

More damage Bloodthirster

More survivability Randuin's Omen or Banshee's Veil

If you want more survivability check whether you're getting more physical or magical damage. If their team does more magic damage, buy Banshee's Veil. If they're doing more physical damage, buy Randuin's Omen.
Examples:

• The opponent's team has an Annie with 12 kills and 2 deaths, while their Ashe had 0 kills and 10 deaths? Buy Banshee's Veil.
• You're up against a team with Xin Zhao and Caitlyn who keep killing you, while their Karthus doesn't even manage to scratch you? Buy Randuin's Omen.
  
  

  Quick summary:

  Follow the item purchase sequence at the top of this guide. Remember to buy 1-2 Health Potions and 1-2 sight wards whenever you have extra money after your shopping trip.
  If you want more survivability, replace the second Bloodthirster with Randuin's Omen or Banshee's Veil

• Level 1, put a point in Broken Wings
• Level 2, get Ki Burst
• Level 3, Valor

After that, in order of importance, max:
R > W > E > Q

These are some of the updates I've planned to make the guide better. I added this section to show you that I'm intending to work on this guide to make it the biggest and best Riven guide around.

[*]Update the formatting of my guide. I spent a long time making the guide look better and making it easier to read, but it still doesn't look like I'd want it to. More color, headings, pictures etc. Still looks like a wall of text at some points.
[*]Add a list for Pros/Cons of Riven and this build
[*]Add a map for ward placement(!)
[*]Elaborate more on my item selection and why I purchase them.
[*]Add a section of alternative items you could build. And a list of items not to buy, for example Trinity Force and Tiamat
[*]Explain my skill order and add information how to use skills / tips and tricks
[*]Tips on teamfights
[*]Tips, tricks and tidbits I've learned for Riven
[*]A quick TL;DR summary of the build in one paragraph
[*]Pictures and possibly videos