Renekton is not the early game lane bully he was. He is now the mid game god bully. I don't play for early game as renekton, I just run teleport and farm. When you have a champion who can trade as easily as renekton, it's really easy to make it to the mid game. His toughest matchups are gnar and vlad, but both matchups are incredibly winnable. The standard e w auto Q e is what keeps you alive in the lane until your Q levels up and you start winning trades, if not already. You e and w without auto because gnar and vlad will both pool or hop out because they know they will lose the trade if renek does stun them.

For runes, I run scaling armor and scaling mr, when you reach level 7 they both outscale the flats and post level 6-7 is when renekton really starts to shine so it makes sense. You're Q is SO much stronger than the previous one, so once it gets 4-5 points, you've got amazing sustain. Unfortunately Vlad's cooldowns are much shorter so you'll have to call for ganks you can't out sustain him, but what you can do is make him push then go in on him when you have relatively even health and all in him, by that I mean pop your ult, he will be forced to run because he won't out trade you.

Against gnar, it's a skill matchup, same like vlad except gnar has no innate sustain, so at a certain point you can go in and chunk him and hop out, try to hide in bush and pop out for cs, he might ward the bush but that makes him vulnerable for ganks. When the lane is decently pushed, because it will be pushed (don't push the lane against gnar or vlad unless you're resetting the lane or you're stronger than them) just all in gnar, pop your ult and make sure you land your e. Maybe gnar will survive but he has no sustain so any quick trade you make will be in your favor.