Isn't the only argument to be made for any of those players is Malkin has the best playoff run of the era even though Crosby was closer to Malkin than their point totals would indicate that year based on matchups?
I guess Keith may have had a better run than Crosby's best too but do Kane or Toews have a better Cup run than Crosby's best?
Crosby has the clear best playoff point totals and PPG of his era, and the clear best Cup run resume of the group (arguably could be Keith). He has four Cup runs where:
(1) Lead the playoffs in scoring
(2) Set an era best goal total of 15 (since tied by OV)
(3) Won the Smythe albeit a weak one but still was determined to be the best player on a very good Pens team
(4) Won the Smythe with a notable SCF performance
It is good that you note that talk of bad defensive teams (a debatable argument to begin with) has been put to bed with recent performances.
I don't want to get too much into comparisons with a bunch of ineligible players, but briefly...yeah I think Kane/Toews 2010 and 2015 are comparable with Crosby 2009 and 2017. 2013 for them and 2016 for Crosby are also reasonable comparisons. I think Keith was probably all around a little more consistent than Kane/Toews and has the best Conn Smythe performance of the last 15 years in my opinion, so throw him in as well. In any case, it's not a run-away for anybody. Which is fine; I don't think any other candidates can decisively claim to be the best of their era either, so it's not a strike against Crosby.
Based on the GA finishes of Belliveau's and Hull's opponents (e.g. 1, 2, 3, 4, 5, or 6):
Beliveau's opponent's averaged a 2.9 finish.
Hull's opponent's average a 2.5 finish.
Translating this to the current era (e.g. a 1-5 finish in GA = an O6 1 finish in O6 GA, a 6 - 10 finish = an O6 2 finish in the O6 era, etc... ) Crosby's opponents averaged a 2.6 finish.
Interesting to see that's how Hull and Beliveau fall compared to each other. It perhaps illustrates the difference between Beliveau playing the Hawks and Hull playing the Canadiens.
I'm not sure how well the O6 numbers can directly translate to eras with more teams in the league. 6 teams is problematically small when trying to scale to 30 teams. To clarify, my original methodology was meant to provide context to a player's raw scoring totals when compared to other players from that same era. It wasn't an attempt to normalize scoring totals across eras, though it may we a worthwhile endeavor to explore.
The issue I see with the methodology employed in the bolded above, is that a 30-team league has 5 times as many data points and is likely to provide a more consistent and accurate measure of defensive strength year over year, when using GA as the proxy. A 6-team league can lead to situations where minor differences are hugely exaggerated by the points system you've established.
Take 1963-64. You have Montreal (167 GA), Chicago (169), and Toronto (172) leading the way. Scaling this to last season, 1964 Toronto would be assigned the same defensive value as the 2018 Avalanche. The 31 data points that saw Colorado end up as a slightly above average defensive team have painted a fairly accurate picture. The 6 data points that have suggested Toronto was similar seems far less accurate. Was league-leading Montreal also just a little better than average? Or was half the league very strong? Neither conclusion seems appropriate. It's a problem that I can't really offer a clear solution to. I think you almost have to look at multi-year sample sizes to establish a baseline for O6 years and grade them on a more finely tuned scale. 1-2-3-4-5-6 is just too broad to be a reliable indicator.