This is my 1st guide on mobafire, so please do comment on how and what to improve.
I really enjoy Riven and after some non stop testing this is what my build turned out so far.

I know she's new and everyone has their own opinions about her.
But give this build a go, before you say it's horribad.
The build really does work if you follow the few guidelines that I will give you.
It won't make you a better Riven player though, so if she's not your thing. I'm sorry~

Please! If you can't be bothered reading, the few guidelines to why I pick that skill order and early + core items. Then please do not use this build and say it sucks.

The way I build Riven is golem/tanky-dps, name it anyway you want.
This build was inspired by Renekton mainly because Renekton has to deal with comparable issues.

This build is not the tons of damage build, refer to other more squishy Riven guides for that.
What you will excel at with this build is, early game harass and awesome sustain/farm.
Growing stronger and stronger into the game until you become a complete & sexy monster.

Solo top or mid, depending on your opponent and yourself, are the lanes you want to pick.

Refer to the item build in that order, at least up to Atma's.
And above all do not ignore the maxing of E 1st in the skill order.

Have fun playing Riven, as I sure am ~

For runes I pick:
Yellow: Armor x9
Red: AD x9
Blue: CDR or MR (this one still up in the air for me, leaning towards MR more)
Quint: AD x3

I think only the AD needs an explanation here.
AD gives you early game advantage over ArP, and for end game you'r building Last Whisper so that ArP wouldn't be such a big change.
On top of that all Rivens skills (including her passive) scale of AD with good ratios.

Flash + Exhaust are my spells of choice and by now I don't think those need much explanation.

Other spells to consider
Ghost
Cleanse

This part is very IMPORTANT!
A lot of people consider maxing either Q or W 1st and only use E for the short dash
Don't do this!!!

Maxed E combined with Regrowth pendant give you better sustain then most supports could ever dream of to provide early game.

A little comparison of your shield.
A health potion recovers 200hp over 20 seconds.
At lvl 3 Valor (which you ll have at lvl 5) provides you with a shield that blocks 100+(15AD, from runes only)=115 dmg shield & dash with 9(at lvl 3) sec cooldown you can activate 2 valors in 1 pot duration blocking up to 230 dmg in the same time a full pot runs.
Whatever little early game dmg gets through your shield, gets recovered back up by your pendant.

So Skill order: get all 3 then MAX E!!!!!!!!!!!!!!!!!!
Q,E,W,E,E,R,E,Q,E

Early on you can enjoy easy harassing. Close the gap by using your full 3 Q combo.
You can simply close the gap with the target by doing 2x Q and landing the 3d on top of them, giving you a free auto attack hit after you knock em up, then stun em with W hit once more and dash out with E which will block projectiles and minion fire.
Wait for your short cool-downs and repeat.

Be creative though, you can dash in with E blocking initial hits then hit, stun with W hit again and get out with Q

Get in with 1 Q, dash with E, stun with W get out with the rest of Q

Keep the enemy guessing and keep getting your last hits

Ultimate
Rivens ultimate deserves some attention too.
I hear a lot of complains about it, but gosh that skill is amazing.
Longer range on auto attacks AND skills!!! Your aoes suddenly don't miss anymore and that 1 step too far is suddenly not needed either.
MORE DAMAGE! on everything, skills AND attacks!
Long range cone attack as a cherry on the pie that hits everything in its path for good to very high dmg!
AAAAAND!!!! DID I SAY: YOU CAN KEEP IT UP ALMOST NON STOP AFTER LVL 16!!!
It lasts 15 seconds (with 40% cdr the cooldown would be 27 sec at lvl 16)

Read the Skill Sequence to understand the synergy of regrowth pendant with your skills

Start
Regrowth Pendant + Health potion

Early
2nd Regrowth Pendant & Merc Threads
Followed by Phage
(although don't be surprised if you end up farming straight up to Frozen Mallet or beyond)

Mid
Frozen Mallet
Atmas Impaler
After that frozen mallet is done you'r scary, there's no easy running from you without wasting flash and even then it's not guaranteed a safe retreat.
You will be chopping at opponents doing your short interrupts using that passive while still blocking quite some dmg with your shield every few seconds
And once that Atma's is done things will only get better.

After Atma's your core build is finished and you will have to decide for late game.

Late
Make a choice.
Are they not doing much magic dmg grab a Warmogs
Are they really not doing dmg grab the Last Whisper or IE even before that
Is magic dmg causing you trouble get that FoN
If you need more dmg rather then tankyness then Infinity Edge over either Warmogs/FoN

Pros:
NO mana/energy!
Amazing sustain!
Amazing farm!
Amazing harass!
Slows/Exhaust can't slow you much!
Very good at juking!
Teamfight aoe disruption!
Sexiest Bruiser out there!

Cons:
Everyone wants to play her ~
Possible to get overconfident
Good timed silence can screw you over

Riven is one of my new favorites, just for the fact how fun she is.
I hope you guys will enjoy playing with her as much as I do.

If you like this build please feel free to leave a comment or vote it up.
If you think this guide can be improved let me know and I ll work on it.

Riven is new, Riven is fresh and I'd be glad to find out the best build for her too, so please do leave your thoughts behind ~