renekton has extremely clear weaknesses... i'm a renekton main since season 2. He is like tryndamere, deny him fury and he is useless. Let him stack up fury, you are as good as dead. Also his dash has an extremely long cooldown (17 seconds) at rank 1 which he won't be leveling before level 8. Deny him fury, call your jungler and force him to use his dash aggressively, he is dead or if you are both braindead he will escape with flash. Also there's an item in the game that goes by the name ninja tabi that screws his W damage.
If you are playing a ranged champion like quinn, jayce, gnar (not the best example) consider purchasing an early executioner's. Most renektons will spam E Q E out if you get an auto while he is dashing you deny him 50% of his heal.
There are more weaknesses but i have to crush a lane with my beloved renekton atm