Renekton
Look no further than c9 vs fox today and you will see... renekton is an early game "bully" in name only. When faced with a champ that is capable of dueling... even in the strongest portion of the game for renekton, it is a pure skill matchup at best. Huni picks up a solo kill because licorice plays too agro. Then he gets a reset to come out slightly ahead thanks to a repeat gank from his jg. From here on huni cannot even contest 1v1 away from his tower although they are more or less even... he just keeps dying and falling further behind. This would have happened less quickly w/o jungle assistance, but it was pretty clear it would happen all the same unless licorice severely screwed up 1v1.
But shouldn't you automatically win if the other guy messes up even a tiny bit or is slightly behind? No. The strength that champion is SUPPOSED to have at that point in the game also should matter. Like if jax or irelia or fiora have 4 items to renekton or darius with 5 items... I give the edge to the former 3 vs either of the latter due to scaling. Yet when faced with a parallel situation early... we see no such advantage for the "lane bully" early game champ (darius, renekton). That is a poor job of balancing champions. Either their supposed innate early game strength should let them still win in situations where they make a very small mistake or are behind very very slightly early on... or their lack of ability to win in such situations should be mirrored by a jax, fiora, or irelia (just using examples here) being unable to win if THEY make a similarly small mistake or are slightly behind late. But that is nowhere near the case; thus, rebalancing of champions is needed.
I've mained renekton for 2+ years now and am around top 10,000 world, top 1000 NA. Still climbing to my proper elo and still being regularly target camped... so that could fluctuate. That might not sound impressive, but look up your own best champ on professor.gg and then reconsider. Besides that... rank does not mean everything. We've got rioters in all elo's making decisions about the game. Some of the pro coaches are not anywhere near top 100 or even challenger, but their intelligence is respected. I could give a ton of other citations to back up my authority on the matter (I'm not an expert on all things lol, but in this matter I do believe my opinion holds more weight than my ranking would suggest). So...
It is my opinion that renekton's kit is wonderful. His base abilities synergize beautifully... and are one of the two reasons he is not in the bottom half of toplaners for winrate right now. The other is essence reaver... an quasi-broken item which we know lends incredible power amplification to a champ's kit if they can make use of it.
Renekton's basic abilities work beautifully together... but... they are only wonderful together. This means they are gated by the longest cd spell which is needed to engage your opponents... your e (dash/slice-n-dice). This is on an exceptionally long cd compared with many other champs' engage tools as the game progresses. In the early game it's not horrible, but the effects are felt very much in the later game. Because of this... it makes the rest of his basic abilities less effective than you would expect for their shorter cd's.
Renekton lacks innate damage amplification, his passive is weak beyond early game when you can control the minion wave to stack it up, and he additionally lacks innate durability amplification aside from pure hp via his ult (which we know is the weakest durability stat). What do I mean by this? Let's look at some examples...
Fiora - passive provides sustain, deals bonus damage and scales up with purchase of additional ad; this is different from a mere situation of buying more ad gives me more sustain and damage or rather is an additional amount of it which scales better due to the %hp damage and lack of meaningful cd on her passive (she already gets the same damage amp a jax or renekton or tryd would get on their base abilities and autos... her passive just takes it to another level)
Mundo - gains tenacity with his w, gains stacking mr with each time he uses spell (up to a cap... that increases throughout the game), e damage is increased not by just a flat amount but also based off missing hp
Jax - attack speed is high, this is made higher still w/o items via his passive; attack speed is one of the best scaling stats... as we can see anytime we watch pro play late game where adc's (who have high attack speed) reign supreme even in most cases if dueling a bruiser (who typically relies on spell-casts which gives windows of burst and therefor falls off), gains significant free damage via his ult's passive even if he builds full tank, ult also makes all his damage items perform double duty thanks to the resistances he gets from them via his ult
Garen - w provides tenacity and significant damage reduction regardless of itemization, gains true damage against the "villain", ult execute makes him more relevant in teamfights even when behind and going full tank; his damage must be respected
So how does this differ from someone like renekton? Renekton passive doesn't amp damage the way fiora's does at any point of the game and it has a meaningful cd as you have to stack up your passive again. So his is more situational, only good at a certain point in the game too, and lacks the same scaling even if the former 2 issues didn't exist. Similarly, there is no attack speed or ad steroid in renekton's kit like garen gets against the villain, mundo gets for losing hp, or jax gets via his passive (and ult passive). Thus... hopefully you can see he lacks the same type of damage steroids that other champions get which help them scale better regardless of their itemization. Similarly, renekton lacks meaningful durability steroids like garen's w, mundo's w, mundo's e (stacking mr) or jax's ult. While you could argue his ult kinda serves that purpose... you'd be partially correct at best AND hp is the weakest form of durability enhancement for tanky champs.
Now I'm not saying renekton is garbage tier and buff him so he can braindead 1v1 anyone at any point in the game. But I'm arguing that his kit lacks the base stats to offer him the same power advantage early that scaling champs like fiora, irelia, or jax enjoy over him or darius or other similar champs mid/late game. And the only reason this isn't abundantly clear to all right now is due to essence reaver which kinda breaks him. His winrate isn't high enough that everyone's spamming the boards with "gut renekton" right now, so he looks to be in an ok spot. But consider that... with essence reaver his spells are on a cd more comparable to yasuo or riven rather than the cd's they are "supposed" to be at with 40% cdr. And that ONLY with this does he manage to have a competitive winrate. A champion's balance should not revolve around an item; the reverse should be true. And the early game power of such powerful scaling champs like irelia really needs to be looked at... such that early game champs have a more meaningful advantage over more scaling oriented champs in the early game; comparable to the advantage a scaling champ enjoys over an early game champ come mid/late game. If such a comparable advantage is not given to the early game champ in laning phase, then they should be given more comparable scaling instead... such that the advantage enjoyed by the scaling champ in late game is reduced to a more appropriate level.
Putting that last idea simply... If renekton's power early is an 8 vs the 4 of a jax early... then the power difference is 8-4=4. Thus the power difference late game should also be around a 4. So if jax's late game power were rated at 10... a renekton's late game power should be around 6. I can understand a +/- 0.5 in power rating... but being off by over 2.0 from where it should be to keep the power advantages comparable would be unacceptable. And that's kinda where it's at with some champs compared to others. Not just renekton... but other champs and other roles too. So honestly... look into balancing champs better - especially the power curve differences between some scaling champs and some more lane focused champs or champs who are just ok at all stages of the game please. And balance champs around their peers and not some item. Items should be balanced around champions after champions are properly balanced among themselves; not the other way around. Seriously... you guys have done SOME really good things. Not everything... but somethings. However, there's still work to be done.