John Nicholls won the John Nicholls medal 5 times. He is the last captain-coach to win a flag. He was first choice ruck for Victoria basically from 1960-1970. He is probably the most influential player on the field I have seen.
Norm Smith said Jack Dyer was the greatest player he had seen. So did plenty of others. Not Skillful? After he slowed down a bit and Bill Morris took over first ruck for Richmond, Dyer played as permanent forward and kicked 46, 64 and 45 goals in his last 3 seasons (although his best goal-kicking performance was 9 goals in the 1944 Prelim). Also - he invented the drop punt.
Bruce Doull - I presume you never saw him play. I'm a Richmond fan - I hate Bruce Doull! - and he was one of the greatest players I have seen - defender or otherwise.
I saw both Doull and Nicholls play. They were great players but not top 20 of all time. Nicholls record is great but compare it say to Gary Dempsey - 7 B and Fs and 13 top 10 finishes in the Brownlow, and I don't think Dempsey quite makes it. Doull is closer but I think the simple fact is there are a bunch of midfielders and forwards that make stronger claims. I think Peter Knights and David Dench were far better footballers than Doull but I wouldn't have either of them in top 20 either (though Knights is close).
Dyer, you could be right on him. I have not seen much footage other than the famous whack on whoever it was. Hard to make calls on players you know only by reputation.