League of Legends (LoL) Question: What side of Kayn?

Most of the games I've played, the Kayn players are all using Shadow assassin and with lethality, its really just easily killing everyone. I haven't seen any Rhaast at all since rune reforged.

From my experience; It depends on the other team's comp and your own team's comp. Kayn is very versatile, Darkin can be tanky as well as a tank buster while Shadow Assassin is a glass cannon. If the enemy team has a lot of tanks, going Darkin will be a good counter. If the enemy team has a lot of squishies, Shadow Assassin will allow you to one shot most of them. On the other side: If your team has more tanks (Tank top and supp) going shadow assassin for more burst damage wouldn't be a bad idea. If your team has a lot more squishies, going Darkin will give your team more tankyness.
It all boils down to what will help your team the most.
The more you play him, the more you're learn what's better for any situation.

They're both good. Shadow Assassin is really mobile and can one-shot a lot of squishy champions if you build full lethality. Rhaast is tankier and has a knockup, so he's better against tankier enemies or just peeling for or setting up for your teammates.

I generally play Shadow Assassin when I play him but they're both viable. I would say it depends on the game, unless you're really successful with one form and not the other.