Writing about the builds, matchups and runes took hours so I will write a more detailed guide some other time. I will also add more synergies and elaborate more on how to play in top, mid and jungle respectively.

This guide was made on Patch 13.1 and I will most probably update it every patch.

Renekton is easy to learn. Your only worry should be controlling your Fury. You need to make decisions about which ability you should use with Fury.

Q is your main damage source in the early game but falls off and is useless against tanks.

After Bork, W becomes your main damage source and your burst. W applies on-hit effects at a 100% efficiency.

Using E with Fury removes a lot of armor from your target so keep that in mind.

Your ult is a very strong ability that grants Fury generation, AoE damage and instant bonus HP that counts for Sterak's and Titanic Hydra.

Keep in mind that below 50% HP, all of your Fury generation, even from your abilities (including ult) is increased by 50%.

This build let me climb to Diamond 1 on my main account and Diamond 4 on my alt. The higher your elo, the less efficient this build is as your enemies learn to stay close and CC you efficiently. I was able to reach rank 4 on Renekton in the Turkish Server on my alt with 68% win rate.

I hope I do not sound like I am boasting...

More info on armor reduction: The second cast of maxed Fury E also REMOVES 35% armor from the target. If we calculate for 100 armor, Fury E + Serylda's + Lethality leaves your target with 7.5 to - 6.5 armor at level 18 depending on lethality. This essentially means your target takes 93.02% to 106.10% damage from you. While this may not seem much, please note that a target with 30 armor would take only 76.92% damage and a target with 50 armor would take 66.7% damage. You are essentially able to deal 50% more damage to the target not accounting for the 15% damage increase from Prowler's claw, 12% damage increase from PTA and 11% damage increase from Last Stand. If my math is right, your maximum damage capacity is 144.10%. You may also build Black Cleaver, with also reduces 30% armor from the target. If my math is still correct, in the most optimal case you will leave your target with -20.15 armor, which along with items and runes lets you deal 154.77% of your damage to the target which essentially doubles/triples your damage output against a target with 100 armor.

Edit 1: Moved some text around for better readability.