I personally think the correct Riven build looks something like this:
When ahead: Goredrinker > deaths dance > optional MR > Hydra When behind or even OR facing super tanky team: Goredrinker > cleaver > DD/Hydra/Maw depending on gamestate.
My thought process is when you're ahead, it doesn't make sense to build 3400 gold into more damage as you're already overkilling people since you're ahead. Instead, you want to build resistances to make it harder for enemy team to shut you down but still have enough damage to kill everybody.
This is my take, in my opinion not much changed since season 12, but I do have to test a lot more when I get the chance.
By - AloisNL