League of Legends (LoL) Question: The best build.

There are no such things as "best build". Build will depend on matchups, how the game is going and the enemy team composition. FORTUNATELY, you have stumbled upon a website that has the answers you seek. You can type "Riven" in the searchbar and a number of guides will pop up. I recommend LimitlessHavoc's one, it appears to be the most complete of the bunch. You can take a read and learn everything you need to know about Riven right there. Same goes for Warwick, just type it in the searchbar and a number of guides will pop up, you can look at some of them and decide which one suits your playstyle the most.

Also, just a warning, there will always be trolls, so i would suggest carry champions (e.g Riven and Warwick, yes, but some champs are better with more skill, so if you're new to riven, i wouldn't suggest playing her in ranked unless you're comfortable with animation cancelling and quick combo etc.)