Leave me out of this! Oh, ok, since you've asked.....
I think in reality it is impossible to compare riders of different eras and say riders in the 80s were better than riders of today or vice versa, even more so to compare riders from before the War or the 50s and 60s. Personally I can only go on what I have seen from 1960 onwards and on written records and oral testimony from people I have spoken to for pre1960.
Just going on my own eyes and riders I have seen, I would always put Ove Fundin at no. 1. I hardly ever saw him race a bad race. He was skillful, determined, ruthless and had that indefinable something that makes a great champion. On his day he could (and did!) beat any and every body. I know riders like Mauger and Rickardsson have had better records since but they didn't have the likes of Briggs, Moore and Craven to contend with, all of whom could well be included in a top 10.
And that is, of course, another aspect to this. The opposition. As has been said, in relation to Greg Hancock, although you can't deny his amazing record, he never quite made the heights when Rickardsson, Crump, Pedersen and Gollob were at their peak. Yes, he's 44 now, but where is the opposition?
Although I didn't start going to speedway until 1960, some of my family were fanatical speedway supporters already, especially my two uncles, who had been going since before the War. They always maintained that Vic Duggan was the greatest ever. One of those uncles, who is now 91 and still goes to Lakeside occasionally, still mantains that view!
And then there was Tom Farndon......
I think it all partly depends on what era you were brought up in and who your own personal heroes were. As the saying goes, there is no right or wrong answer.