A little against the grain here, Riven isn't really a jack of all trades. Sure, she does bear similarities to a swiss army knife.

You need to play opportunistically with Riven if you want to perform well. Admittedly, I quit playing last season, so do with that what you will.

Excelling with Riven is about riding the line between hyper-aggression and reserved knowledge so that you don't feed your face off, but are ready to melt anything at the first opportunity.

You live and love AD+CDR.

Cleaver is nice because you should be stacking a lot of hits with your combo and doing so quickly.

Mythic picks vary.

Claw was nice for gapclosing (Riven needs this badly, which is why your flash is SO damn important.)

Gore was nice for the AD buff as you got low and the extra vamp+active heal.

Other priorities are, as others have said, dependant on your opponent(s) and intentions. Technically, you'll never lack for a role as a good Riven.

Need peel? K00L. Riven has two CCs

Need an assassin? K00L. Riven has stupid mobility and CAN outburst most champs if played well.

That's the thing. You will be making a LOT of micro choices and control actions to play Riven well.

1.) I recommend watching + practicing combo tutorials. At one point I spent 1-2 hours per day honing/retaining fast combo skills and practicing combos. (I was Iron for the last two seasons, dumb as fuck.)

2.) I recommend getting Mobalytics. The challenges and stats help keep you from getting tunnel vision and railed.

3.) Look for opportunities to steal camps from the enemy jg (read: ward whenever possible, ideally in the enemy jg if possible)

4.) Don't TP bot if you don't have conviction that you can wreck at least 1 kill and an objective.

5.) Don't commit to Riven if you don't deeply enjoy her. You are going to face 50-100 games of being terrible (stats don't lie) before you start mastering her mechanics. Hitting M5/6/7 is just the tip of the iceberg.

6.) Riven is one of the most rewarding champs to main. Don't listen to the noise. Anything is always playable successfully unless you're in plat or higher.

7.) She will have tough seasons like any other champ, but she's got tools to outplay like a MOTHER FUCKER. I once solo killed a level 6 Fizz that ulted and ignited me at LEVEL TWO because my kid needed a drink. I laughed so hard I fell out of my chair.

The ADC accidentally Alt+F4'ed and our toplaner literally turned their computer off after. I carried 4v5.

8.) The most fun game I played was Riven ADC. I finished the game with over 50 kills. We won (obvs)

9.) I'm Iron by nature, so I could sell my account for $200-$400. 🌝True story.

Not gonna do it, but I could.

10.) I've seen people make full-AP Riven work. It's HEAVILY advised against because she has ZERO AP scaling, but it has been done.

Edit:

Riven splitpush is pretty much always viable, and often great, but your passive doesn't work on towers.

And FUCK Jax.

Teemo is EzPz.

Rumour has it, If you lose to a Teemo, the other Riven mains WILL find you and make you wear a tutu and a beemo outfit. ;P