Warning won 2 Group 1s (his Sussex Stakes from Then Again who retired a maiden, and his QEII from Salse who Sir Henry finally coaxed to win a weak G1 in the Prix a Foret) to Canford Cliffs 5 - I struggle to see how that makes him the superior horse?
Zilzal also won only 2 Group 1s so has already been surpassed by Frankel.
On bare form none of the horses Zilzal beat had the same form in the book as Canford Cliffs - Polish Precedent raced only 6 times and won the Prix Du Moulin (G1) from Squill (G2 winner) and Cadeux Genereux (a sprinter who didn't stay the mile on Gd/soft). He also won the Marois (not sure if it was G1 in those days) from French Stress (a G3 winner) and Magic Gleam (won a G2 race). His other 3 wins were 2 G3s and a stakes race.
Probably the strongest horses Zilzal faced was Barry Hills Distant Relative who short-headed French 2000 GNS winner Linamix in the Moulin and Markofdistinction who won a QEII. But the fact that the horses vanquished by Zilzal and Polish Precedent were constantly beating each other tells me none of them were high class animals of the calibre of Canford Cliffs.
For me Canford Cliffs defeat of Goldikova at Ascot was his greatest performance and he came to Goodwood in top form, yet simply could not live with Frankel. Whether you believe the Ballydoyle excuses or not he was pulverised that day.
I was around at the time of Zilzal (I was born in 1968) and whilst I remember his victories he came nowhere near to delivering the performances we have seen from Frankel. Comparing horses across generations is pointless anyway in my book (ref Arkle ad nauseum).