I know there will never be an answer and there is a group of Letsrun posters who see Ryun as immortal, but I don't think he'd be the best ever. He peaked early and was a true physical freak talent. He "looked" stronger and more fit than most other 1500m runners. When I first saw Alan Webb he reminded me of images I'd seen of Ryun.
Alan Webb, to me, was the late 1990's / early 2000's version of Jim Ryun. Freak talent. Muscular. Super good leg speed. Hard worker. Webb ran 3:30 and 3:46 (mile) and 1:43, all in a month span and never found that again. You could easily argue he peaked early as well. He was very much like Ryun and so, maybe Ryun could have been as fast +/- as Webb was.
There are no white boy USA runners who have ever run faster than 3:30. Why would Ryun suddenly be the exception? If you think Ryun is more talented than jakob, do you think Alan Webb is more talented than Jakob Ingebrigsten? He might be physically, but that's not all is takes to be the best.
Kip Keino broke the 3,000m WR by 6 seconds and broke the 5,000m WR as well. He beat Ryun by 20 meters in the 1968 Oly final...biggest winning margin ever.
Are people arguing Keino would be better than Bekele and Cheptegei?! Or the 3,000m WR holder? I doubt it.
Another thing hard to quantify is that Ryun didn't think anyone could run as fast as Keino did in Mexico City to win the gold medal. He thought a slower time would win. Perhaps this was a mental limitation that prevented him from setting higher goals.
Ryun ran 3:51. Obviously, that'd be faster on modern tracks and with better shoes. So, safe to say he'd have been under 3:50 for the mile. I don't find it difficult to make an argument that would have happened. Today, running 3:50/51 is fairly common. Even Galen Rupp did it. There are guys running 3:49 who aren't sniffing 3:25 in the 1500m. Even if Ryun ran 3:47-48, I don't think he's faster than the everyone else. Webb wasn't and he ran 3:46 and 1:43 in the same month.
El G was the best and most consistent 1500m runner ever, and no one really comes close.