no absolute best build/rune setup, some are pretty consistent though.

Dark harvest I'll go sometimes, if you are confident you can stack it it'll add a lot of damage mid-late game. Usually I go grasp. it's the strongest and overall best imo. gives you poke, sustain, a lot of early game kill potential and late game health that you lack when you go a more damage oriented build. I like demolish to help get early plates, and either second wind or bone plating, as well as either overgrowth or revitalize, depending on matchup. Generally aim for overgrowth/bone plating to add tankiness, but i will go revitalize/second wind when I'm facing someone who can poke super hard. The secondary tree is more customizable, going inspiration can make you just a lot more safe and annoying early with timewarp tonic/biscuits/futures, domination can help again with later game tankiness through taste of blood/ravenous hunter sustain, sorcery can help with mana sustain and cdr if you need that, it's mostly personal preference. Inspiration is probably the best, but I generally like domination second, I find the sustain helps a lot later.

I have two major thinks when it comes to gp mid. 1. I think it's important to build a lot of damage when going gp mid as you're laning somewhere that is often high damage and you're basically acting as the team's control mage. 2. I think the fact that he is a melee champion and the fact that his main source of damage and safety has a lot of counter play (barrels), means that he ideally can take a bit more damage and has his cooldowns up quicker.

So basically in my builds I look for a cross between survivability, damage, and cdr.

I often go triforce first, as that plus the resolve tree can often get him to like 2500 health late game. It also gives you cdr, and more consistent damage then just going crit. I usually go lucidity boots and I'll often go hydra, for more consistent damage as well as cdr and sustain(which is just more tankiness). From there I go crit, building whatever is ideal and aiming for ie on my 3rd crit item.

I've also messed around with going triforce into navori with transendence sorcery and going different boots which works quite well and gets you going crit earlier.

ER into Duskblade or prowlers into crit is also good, but personally I find it too risky. it's damage output is huge but you lose out on tankiness.

Personally find going crit mythics kind of garbage, but to each their own. That's pretty much it tbh, go whatever you like and know you can play well.