This is my first build guide so I did it on my favourite champion. If there are ways i can improve the quality of my guide please let me know.
This build gives you high early game damage as well as building towards high late game damage.

Pros:
- Good early lane harassment
- Strong farming
- Good solo or duel lane
- Good 1v1 Champion

Cons:
- Susceptible to good cc
- If focused in a team fight isn't incredibly difficult to kill
- If dash ability is on cooldown no viable escape mechanism
- Silence is very powerful against Renekton

The 23-6-1 set up does give you some heavy damage bonuses and some survivability with the MR and Armour as well as the ghost buff.
I chose improved exhaust because there is no point getting AP as only his Ultimate uses 0.01% AP and smite is only useful if jungling (obviously). Getting the 15% Magic Penetration is useful for when you have Madred's because that is bonus magic damage. Armour Penetration goes without saying as do the Attack Damage, Attack Speed and 5% increased damage. Choosing the Critical Damage is more useful than any other masteries available with 6 in Defence and 1 in Utility.

The Armour and Magic Resist masteries give you the obvious defensive bonuses allowing you to be more aggressive.
And Ghost is useful to escape unwanted confrontations.

Armour Penetration Runes are exceptionally powerful with Renekton as his 3 main abilities deal physical damage. This means that being aggressive early game to low defence champions is a very good idea.

Flat Armour Runes give you some early game protection from enemy attacks meaning that being aggressive doesn't leave you with really low hp after a few minutes.

Magic Resist per level runes give you early mid and late game protection against most abilities, again giving more reason to want to be aggressive.

Getting a Long sword and a health potion to begin with is strong and gives you more survivability than most melee champions due to the heal from the Q ability. Armour Penetration and Damage makes Brutilizer a very good early game item. After getting Brutalizer and some optional health potions Ionian boots of lucidity are very powerful for Renekton. Cooldown Reduction enables Renekton to build up and use fury quicker thereby increasing his damage output. That is one of the reasons for getting Spirate Visage. The others are Magic Resist for durability and increased health regeneration effects. This makes Renekton's Q heal you for more which gives you a big advantage if you can survive fights that would otherwise kill you. Then Last whisper gives you ridiculous Armour Penetration which results in massive damage output. Frozen Mallet is useful for both it's slow and large health increase. And Madred's Bloodrazor gives you some armour for survivablity aswell as huge damage with it's on-hit effect. This on-hit effect is applied by all strikes of Renekton's W skill.

Getting Q first enables you to do good early game farming. getting E next gives you the ability to easily harass the other team. E through the minion wave then Q then E back through the minion wave. Then get W for another harassment option and some good single target damage as well as a stun. Max Q first to increase farming and survivability. Then max W for increased damage when you stun. This makes bringing enemy champions down easier as the combination of high damage abilities and armour penetration is devastating. Then that leaves you with maxing E last which will allow for easier takedowns as you can chase through enemies or over small walls more often. Obviously you should max the ultimate whenever possible.

Due to his damage and durability Renekton can easily fit in any 5v5 or 3v3 team that needs a melee DPS. As he is not weak early game he is able to work well with mot of not all champions in lane. But the best laning partners are either good melee pushers or ranged champions, This is becuase of the high damage output two pushers can deal is very strong in a lane. In team fights Renekton can initiate but is able to deal more damage if he doesn't have to fall back because he was focused first.

Due to his healing ability and not needing mana Renekton can stay in a lane for a very long time. But that does mean you should. If the enemy go back to base, which hopefully they should if you have harassed enough, that is a good opportunity for you to do so as well. This is because early items are very effective on Renekton increasing his damage and healing meaning the edge that your opponents would have had had you not gone back to base is now yours. Renekton makes a very good solo laner because of his healing and two multi-target spells. Also his ability to jump through minion waves makes coordinating jungle ganks extremely effective, because your opponents won't expect the gank if you appear to be behind your minion wave playing defensively. Also this is a very good way to pick off or single out weak opponents by jumping through their front-line, be that minions or champions.

The answer to this is both situational and your choice.
Situational:
In the lane initiating is one of Renekton's strengths, being able to jump through minion waves and hit the enemy with the element of surprise on your side. If your opponents push out from the safety of a turret, with a fair portion of their health missing, initiating is a good idea. Due to your early game armour penetration you are very capable of severely damaging or even killing weakened champions. Also due to your healing ability you are able to jump on the enemy when you yourself appear weakened, this is more effective if you have your Spirit Visage. I in large team fights initiating may not be the best option. If 4 or 5 enemies focus you down you may not survive for very long. If you outnumber the enemy group or appear to have an advantage Then it is up to you if and when to initiate.

Renekton's target in a team fight should be the weakest target, ie. the mage or physical carry. This is because you can jump through the enemy team and land right on top of them. This may also benefit your team as well because a furious Dice will reduce the armour of anyone you jump through thus increasing your teams physical damage. Also with your ultimate active you deal damage to everyone around you, not a massive amount but it definitely helps. This will help you build up more fury to use against your target, such as a furious Ruthless Predator. After the weakest you should precede to take out the champion with the lowest current HP, some champions however have exceptions such as Dr. Mundo with an active ultimate or a champion with Zilean's ultimate on him etc.. This should bring team fights to a swift and deadly conclusion, that or the enemy team runs away.

If you are able to get some very early kills then yes Renekton can have a snowball effect. This brings an obvious question to light. Should you get a Sword of Occult? It is up to you. If you think that you can end the game quickly and decisively then it is a perfect item. But if you are the only member of your team getting fed then the enemy will try to focus you down. That means that the Sword of Occult may not be a good choice and sticking to this build may work out the best. But I say again, it is up to you.

I hope you find this build useful and that you use it to slaughter your enemies.

If you have any problems with this guide please let me know in a sensible constructive way, thank you.

Your's
Hadesash