How to Counter Juggernauts?

The main weakness of juggernauts is there relative lack of mobility. They are all melee, and don't have a significant gap closer. That being said, any champion who can kite well can do well against them.
Ranged champions are easier to kite with while staying out of range of their attacks, and if you can build botrk, the active movement speed steal will also help make kiting easier. Quinn specifically is good if you can land blinds on them, though other like Lulu are viable as well. Ranged champions can also more easily harass melees in lane, and generally can farm more easily against them.
As opposed to ranged champs, Trundle can also do well against them, as his pilar will can prevent being chased, and his ult steals stats, and his q will reduce the enemies attack damage while increasing his own.

Like Dragonrage said, juggernauts are very hard to fight as a melee champion. They might be a lot less mobile then a Riven, or any other full melee champion, but their lack of mobility does not matter much when both champions have to stand near each other to do damage.

If you know you are going up against a juggernaut and can still pick something else, then do so. Most of the time I pick Gnar against the juggernauts because as Gnar I can kite easily and can deal max health damage. Once my rage bar gets full I can even turn in a juggernaut myself! Fiora works as well because of her max health true damage and her ability to completely negate all damage from a Darius or Garen ult.

If you don't know who your opponent is your best bet is to just hang back and freeze the lane near your turret. By doing this you make sure the juggernaut has to over extend to farm which allows for easier ganks. But if you have enough skill as Riven and you can win trades like you said you can then freezing and when possible harassing the enemy may work very well.

Just make sure a Garen doesn't silence you, a Darius doesn't pull you back in and Illaoi does not grab your spirit. These skills are by far the most dangerous as it allows them to deal heavy damage to you.

And by all means avoid giving the juggernauts an early lead. If they do have a lead even a gank from your jungler is not going to help much and will most likely result in two deaths for your team. Sometimes I tell my team not to help out my lane. This way they can win the other lanes while I make sure the juggernaut doesn't push too fast.

I can't give you any information about Nasus. Preseason is not being kind to him at all.

Well, it depends on who you are playing. If you are riven, it's likely going to be a skill matchup to win lane and Garen just counters you anyways. Anyways, if you are having a hard time as Riven, you should try to shove your lane then roam to middle or TP bot to pick up kills. Even if you are a little behind you can snowball your team at this point.

Nasus isn't really meant to win lane anyways. Just go purely tanky and use that TP to help you stay healthy until you are in a good place. If you get too low, go back to base. The wave will likely end up pushing to your tower anyways in this case and the best place for Nasus to be is farming off of turret.

Although a little unconventional Brand or Vayne can be played top if team composition allows, their massive amounts of % health damage are very strong against juggernauts and they are ranged which is also good against juggernauts.