From what I have seen, Greaves looked a better goalscorer than Kane - but I didn't watch him play every game so can't really say. May not have had the passing range or aerial threat of Kane, but appeared a wonderful dribbler and a truly, naturally gifted player. Statistics tell me he did it consistently too, but I won't likely find many clips of him having games he just couldn't get involved in etc.
Had Kane joined Madrid at 24 I expect he would have won European Cups - but I doubt he would done things like Bale did in cup finals.
In terms of raw efficiency, they are probably both 9/10 or 10/10 players. In terms of excitement, I think Kane would be a 7/10 compared to Bale's 10/10.
'For England' is a different thing than 'best English player'. I would (without seeing him play each week) expect Bobby Charlton to top the ranks there.
From my 33 years, Kane, Gerrard, Gascoigne are probably all up there. Should be saying Shearer, Owen, Rooney too - but feel they had spells for England where they didn't perform like the others.
