According to the Doombrew Riven guide on LoLPro, Renekton is the hardest champion for Riven, as he counters her ability to get in and out on small exchanges and can out-sustain her with Cull the Meek.
Link is as follows: http://www.lolpro.com/guides/riven/144-riven-guide-top-lane/tips-tricks-synergies-and-counters
gaming.stackexchange.com/users/29882/Jellee
There are no champs who can really be considered a HARD counter to Riven.
Some champs that do well against her are Olaf, Garen, Renekton, Teemo, Nidalee and Kennen.
Jax and Irelia can do well after a certain level as well.
I'd recommend goldfather8's guide on solomid.net to learn how to play or play against Riven.
He goes into each matchup in detail, it'll help you understand how each champion can overcome Riven.
The main reason why there is no real hard counter to Riven is that she scales really well, has cc, has a shield for sustain, has damage from abilities and auto attacks (can't be hard countered by Teemo blind), and she has high mobility to escape bad situations.
gaming.stackexchange.com/users/29835/eezoh
Teemo - Why?
He can kite ANYONE.
If Riven tries to close the gap between you and her, you can outrun the hopping.
Teemo bullies any melee champion top lane, and even more if they rely on autos for their damage.
Its really easy too, just poke her like Teemo does, and kite her when she attempts to fight you.
Hope this helps ;)
gaming.stackexchange.com/users/29832/SupaTecmag
Olaf, would be the hard counter if there was one, since once he hit level 6, He should never be killed by Riven at all.
You just need to wait for riven to dash to harass you, and E her and walk away, till 6 when she dashes in, you can ult, when she stuns, and then hunt her down, since not factoring in flash, She has no way to stop you from killing her.
gaming.stackexchange.com/users/29833/Zetwolv Uzu
Riven's health regen has been nerfed which makes poking and kiting a good counter if you have some range.
That is, her sustain is lower now.
She can't go head to head after wearing her down.
Even if you're playing Teemo top lane Bottom lane, a typical ADC and support will poke and kite her out of lane.
Patch notes: http://eune.leagueoflegends.com/news/preseason-balance-update-2-patch-notes
gaming.stackexchange.com/users/23311/gbn
There is no "hard" counter to Riven, but some dudes to decent against her.
Olaf and Rumble are pretty much a skill match up, Olaf can beat Riven pretty hard if played correctly, so can Rumble if he gets a little help from his jungler.
Irelia looses early to Riven, but outscales her after 15 mins even if Riven has several kills more.
Malphite and Udyr to pretty good against Riven, and while Malph just outfarm/scales
Riven, Udyr beats Riven if played correctly.
gaming.stackexchange.com/users/29368/Jacimovski
I just smashed a Riven with Volibear.
He has an monstrous early game, and that's what you need to shut her down.
gaming.stackexchange.com/users/41223/Northo
I find that Riven can do at least decently in every lane if played properly, is there actually someone who can counter her reliably?
Is there a HARD counter to Riven?
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gaming.stackexchange.com/questions/78056
gaming.stackexchange.com/users/29829/Feral
Renekton, Olaf, Garen, Teemo, Nidalee and Kennen do well against Riven. While many people can think of some champions that could potentially give hard time at Riven players, it seems that Riven can't be hard countered.
The community suggests few champions that might counter Riven. However it seems that there is not actually, what is commonly considered as, a hard counter.