A random thought sparked off by a recent discussion on QQR. I know it goes against the canon of all Greats being equal, but it's an interesting idea to explore.
Now, why would I say that?
What are the possible evidence of Yideng being slightly weaker than the other Greats?
1. Yideng NEVER once fought the other 3 Greats (on screen). We know the other 3 Greats are equal because we see them fight each other constantly in ROCH and especially LOCH. In fact, Yideng participated in a grand total of... ZERO fights in LOCH.
2. Unlike the other Greats, Yideng stopped actively trying to improve his martial arts to new heights since becoming a monk, pre-LOCH. He's more interested in chanting sutra to redeem his sins. Wouldn't that slow down his martial arts progression (however little), relative to the other Greats?
3. He used up "5 years of internal cultivation" to purge HR poison (although if i recall correctly, he restored these via 9YZJ). Given his benevolent nature, it is likely he would use similar techniques to help other dying people during his travels.
4. In ROCH, Yideng fought QQR and can only "hope to beat QQR by half a stance after 1000 exchange". Perhaps this is actually true - because we know how overrated QQR is! This would make total sense if Yideng actually sits somewhere between QQR and the other 3 Greats. This is further supported by the fact that YG defeated QQR much faster than 1000 stance shortly after.
5. The only other character he fought a full scale fight with was post 16 GWM, and we know Yideng would have lost. GWM never fought HYS. Is it not possible that GWM = HYS, and both are ahead of Yideng?
Conclusion:
Aside from "author intention", there is no actual on screen evidence that Yideng is on par with the other Greats. The only 2 chars he fought with in the whole condor series are QQR and GWM, and we know the performance of these 2 guys.
I would argue that Yideng might actually be closer to QQR LV rather than the 3 other Greats.