Graves is really strong right now. Some people play him full lethality, but I think the tankier version is far better, since you still deal tons of damage without being a glass cannon no one wants in their jungle. For runes take Phase Rush for early game pressure vs junglers you can 1v1 and also enemy laners that would be hard to kill without it. If you already know from the team comps that you are not going to do much early, take Fleet Footwork for faster and healthier jungle clear (+harder to kill when invaded or fighting in river) and just power farm to become the ultimate late game machine.

So far I've played this build quite a lot on my main account and had good success with it in EUW Diamond Elo (Diamond 2 atm) op.gg: demon youth.

I don't know if this is the best build out there but I learned it from Tarzaned, the Rank 1 NA jungler and to me it feels like the strongest out of everything I tried. This is just me sharing my experience with you.

FOR MY EXPLANATIONS ON ITEMS AND SKILL ORDER READ THE RESPECTIVE NOTES ABOVE

My comments on the abilities require you to already know what's written in the tooltip, so read the ability-tooltip first. I will only give additional information, helping you to adequately understand and use each ability.
If you are new to the game and don't know how Graves works at all, just enter a training game, read his ability-tooltips and try everything out for a bit.

Passive New Destiny:

His "passive" is really just an explanation of how his basic attacks work. That's not really a passive. Pretty lame by Riot Games.

Q End of the Line:

If possible, try to hit a wall with it so the projectile comes back faster. Also, when you want as much burst damage as possible (e.g. while trapping in a bush) cast Q first and then immediatly basic attack after that, not the other way around. This will cause your basic attack to kind of cancel the Q animation leading to you dealing more damage in a shorter amount of time after revealing yourself.

W Smoke Screen:

While ganking this is a pretty neat tool for gap closing because it's slow is not to be underestimated. Later in teamfights try to throw Smoke Screen on the enemy AD Carry to blind him, causing him to panic and not being able to deal as much (if any) damage per second on your team. It's not uncommon for enemy AD Carrys to instantly flash out of your W when you hit them in intense late game teamfights.

E Quickdraw: This ability is my favorite. It can be used in so much ways. You can use it to travel over thin walls, you can dodge abilites, you can close the gap between you and your enemy or just kite back a melee while getting an extra slug to shoot. Also its passive helps a lot with taking less damage in longer fights or while jungling. Try to keep E-Passive stacks up while jungling by kiting camps towards the next one and refresh it by using E if necessary to take less damage from camps and also be more potent in 1v1 situations if you get invaded.

R Collateral Damage:

This ability is pretty versatile, too. You can use it to secure kills on low life targets that are far away or just to deal a lot of extra burst damage in close range situations. Also you can use the 300 unit recoil to travel over small walls in emergency situation, as well.

I mostly start Blue -> Gromp -> Wolves -> Raptors (use E in between wolves and Raptors to keep passive up) -> Red -> either Scuttle or Krugs depending on whether your laners got priority and you think whether or not you can win a fight in the river.

To be successful with Graves, you need a very farm heavy playstyle. Also try to counterjungle a lot when you know the enemy is somewhere else. A good balance between counterjungling when the enemy is somewhere else and being at the right place at the right time for counterganking is key to win most games. Also punish overextending enemy laners by ganking but do not try to force suboptimal ganks/fights just to "do something". If you are not sure that the gank will be successful, don't gank, just recall and keep farming. You will win a lot more games like that. Prefer getting a small lead without any risk over a huge lead with the risk of falling far behind.

Thank you for reading my guide. I know that this isn't the most in-depth guide out there, I just wanted to share as much useful information as possible in a compact format.

If you want to support me, please check out my stream:
https://www.twitch.tv/demonyouth77.

Thank you!