Renekton just needs his spells rebalanced. His q does less damage than his aa without items. He deals no damage to enemies with armor values stronger than ninja tabi until he completes cleaver. He does not benefit from the cleaver components nearly as much as others because all he wants early on is DAMAGE. Pure, raw ad. Darius benefits from the components more since he just wants a long fight to get his passive stacked up and active - which hp gives you the ability to do. I don't think jax and irelia are in near as bad a spot - especially jax. He's kinda in a similar situation to aatrox... with people just whining for buffs until he's too op when he's really ok the way he is (still have people claiming aatrox is an auto-loss; open your eyes).
Jax has a range of options depending on opponent team comps and what his goal is (split vs teamfight). While he might not be the top tier toplaner, he's definitely not garbage. Irelia is similar to jax in items and in the runes she tends to take... to a point. She might not benefit as much from some of them since jax gets tons of basically free damage via passive (attack speed), low mana cost w, and passive on his ult (3rd hit proc); irelia needs her w active.
Darius's problem is he gets kited too easily and either deals no damage even when he proc's his passive in teamfights because he can only hit tanks or he deals damage because he has 2+ damage items but is generally focused and killed before he can get his passive stacked up and start ulting. This can be somewhat addressed by either adjusting melee movespeed items to scale off %movespeed (instead of flat; crit gives %movespeed) and keeping that movespeed in combat or by making crit movespeed NOT stack and also fall off in combat (except pd - as it's movespeed unique passive is specific to in-combat) to keep equality with melee movespeed items like righteous glory or deadman's plate.
Renekton's problem is both runes AND his kit. The champ has weak scaling compared to everyone else while currently also needing to OUTPLAY scaling champs in order to win early. To put a comparable example in place - a FICTITIOUS one - that highlights how stupid and problematic it is, let's use the following...
vayne's q is turned into an aa reset that does 2/3 the damage of a regular aa. Her w can only be procced once every 15 sec on a target. Her q scales off ap. Her w scales off attack speed, her e and aa's scale off ad and attack speed. By late game, her ratios, cd's, base damage, and items combine to let her COMPETE WITH (not outscale) other adc's. She needs to outplay a cait to beat her 1v1 late game and straight up loses early/mid by half her hp bar. She's mildly stronger at teamfighting, but not by a respectable enough margin to warant picking her over caitlyn. If such a scenario were true, who the hell would play vayne? Yet this is exactly the way renekton is treated... only the early game vs late game is flipped and of course cait is instead other fighters/bruisers and sometimes even tanks in the toplane.
Renekton either needs to be returned to the early game status bar - if you are good and play properly, you can lane vs him (not win, but survive just fine and outscale) or needs much improved scaling and honestly lower late game cd's more on par with a riven (2-5 sec) not 4-10 sec (it's been awhile since I've played him; I don't remember the exact numbers).
I may not be challenger, but I sure as hell know renekton. I know him well enough I have carried 3 teammates who fed their asses off for 17 min straight while I was 1v2 winning my lane (with tp) vs an ignite counter-pick toplaner more times than I care to discuss. I know him well enough to adjust my builds going into the game based off enemy team comps to maximize both splitpush and teamfight potential and predict which one I will be doing more of. Enough to beat decent jax, camille, yasuo, trynd, ww (jungle), etc. players a full 2 items after I should be losing to them no-contest. The champ needs some love, some updating. His basic abilities are great and have some of the best synergy in the game. But the base stats, the ratios of abilities, the cd's, etc. are not so good. A champ at his strongest point in the game - one made to duel others - should not be losing to other champs at one of their weakest points in the game unless massive outplay is involved.