just going to play the devils advocate a bit and show my Lizarmen bias

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At least you admit it. (we all fall into that sometimes... I certainly do as well)
I agree here, it was partly just to state that comparison in general might not give the best ideas of power level. However even though orcs and indeed black orcs are stronger than humans, I don't know if they are exactly on level with saurus. Orcs might be natural fighters through natural selection and all that, still Saurus are literally made for fighting and have natural defenses and instincts that orcs do not; furthermore orcs can die of old age (to my understanding) so even if Saurus do not fight as frequently, they have more opportunities in total.
The lifespan is definitely a fair point, and something that falls in favour of the Saurus. That said, a Dwarf doesn't have nearly the lifespan of a weedy little elf, but they can easily hold their own.
As far as instincts go, they can only get you so far. Learning from experience will get you to a higher level (if you survive the journey). That's why humans were able to bring down any prey, even though the prey had greater instincts (and I'm talking long before the advent of guns). Orcs fight more often. I'm sure Grimgor has faced and learned from many more opponents than Gor-Rok. Saurus have less major opponents to face and will only fight when necessary. For Orcs, fighting is a way of life. They are battle tested against more significant opposition (think of all the armies in the old world).
Also remember, that there are a lot more orcs than there are saurus. Meaning greater odds of a truly exceptional specimen arising from the masses. Greater numbers means greater odds.
With that in mind I think it can be argued that they Grimgor and Gor-Rok might be about equal in terms of exploits, the key is 'might' since we don't know for certain of course.
I disagree. Of course, each army book is going to prop up their own characters. However, if we move outside of the Orcs & Goblins and Lizardmen books, I think Grimgor is mentioned far more frequently. His impact on the Warhammer world has simply been greater.
Sure orcs have to fight a lot, but Gor-Rok is a perfectly made killing machine right out of the spawning pool, he probably has no need to sharpen his skills as they are natural to him.
He who does not learn and evolve, eventually gets left behind. It doesn't matter what kind of innate gifts you're born with, they have to be honed, progressed and adapted.
I don't know if he is below Oldbloods necessarily, maybe in terms of war tactics but in terms of pure close combat skills I'm not sure. Even in terms of tactics he can't be too bad, seeing as he has led armies by himself and has been granted his own pyramid, something which I haven't heard any Oldblood get.
Page 32 of the army book reads as follows:
In a society such as the Lizardmen, I'd imagine that rank would be given purely based on merit. Unlike in human civilizations, where politics, birthright and personal connections often mean more, I'm pretty sure Saurus ranks are gained based on ability and accomplishments.
We can also see it directly in their generic state lines...
Granted, Gor-Rok, being a special character, will be an exceptional example of Scar-Vet, but if he was worthy, then he'd be an Oldblood.
Beside he has two powerful pieces of equipment, the Mace of Ulamak arguably being a very strong weapon, as it guides his strikes, and he likely does not wield it badly to begin with.
Grimgor also has two powerful pieces of magical gear, and I'd argue that his are far more potent.
His axe Gitsnik, was crafted in Zharr-Naggrund by the Chaos Dwarfs. Nobody creates better weapons in the Warhammer world than Dwarfs (whether regular Dwarfs or the Chaos variety). This is also massively evidenced in the actual rules, where his weapon grants him 2+ Strength and ASF. That is
massively better than Gor-Rok's mace. As for his armour, it grants him a 1+ armour save and 5+ ward. That is far better than Gor-Rok's 2+ armour save and no ward.
For the record, in-game, Grimgor mathematically kills Gor-Rok in one round of combat before Gor-Rok can even strike.
True, but then again rules seldom show the true power levels of characters, just look at Mazdamunid or Kroq-Gar, they are quite pitiful when compared to their lore-equivalents.
I would argue that the rules are
usually fairly indicative of how powerful a character is. Obviously all characters are scaled back in-game (especially wizards) when compared to their fluff, but that holds true for all of the characters, so the comparison is still valid. I'm not claiming that it is a perfect piece of evidence, but when there is a large disparity, then it usually holds true. There are exceptions to the rule, but more often than not they provide us with some sort of insightful information. Now being worth one's points cost is an entirely different matter. A powerful character can be over-costed while a weak character can be under-costed. Mazdamundi isn't worth his points, while Tetto'eko is a steal, but points aside, Mazdamundi's rules to present him as being more powerful than Tetto. If we discount points, Mazdamundi would stomp him.
Your examples you list are fair ones, but that mainly comes down to the fact that ridden monsters are not very good in 8th edition since they can be cannon sniped too easily.
Once again, exceptions do apply, but it is still worth noting.
I've present a bunch of anecdotal and supporting evidence in favor of Grimgor. There is no purely objective and hard fast proof (in terms of fluff), because they have not fought one another directly in a story. As such, there will never be any conclusive outright winner, but the supplementary evidence does seem to favour Grimgor... and it does so pretty significantly. Some of the rationale I provided does have restrictions and limitations (which both of us largely agree to), but
collectively they paint a pretty clear picture (IMHO). What is the evidence in favour of Gor-Rok? Other than having a longer lifespan, he doesn't really have anything else.
Let me leave you with one final point. Ask a a bunch of warhammer players (who don't play Orcs or Lizardmen) if they know of Grimgor and Gor-Rok. I would bet that more of them would know of Grimgor. The reason being is that he is more epic. Ask a non-hockey fan to name a player and they will likely name someone like Gretzky. Soccer... Messi, Ronaldo, Pele. Golf... Tiger Woods. Basketball... Micheal Jordan. Why are these figures more easily recognized? Because they were the best. The same is true in Warhammer. There is a reason why more people know of Settra rather than Apophas; the same is true between Grimgor and Gor-Rok. Grimgor is a major warhammer character (not just a major Orc character), while Gor-Rok is a minor Lizardmen character (overshadowed by Kroak, Mazdamundi, Kroq-gar).