I think Ewing is just simply the worst player here, so he would go at the bottom.
Out of the 3 guys here, I feel like it makes sense to pick them based off how you would rank their offense as a rough estimate. All have their defensive warts, so I will just focus on their offense in the post. I would rank their offense in order: Nash, Harden, Barkley.
Therefore, my list is:
Nash
Harden
Barkley
Ewing
I think of Nash as the best offensive player here, so he would be my #1.
I take Nash for peak and an all-time sense. The only people who have a 8 year string of postseason offense as good as Nash are Shaq and Lebron (via backpicks).
Furthermore, per Backpicks BPM, James Harden has among the greatest dropoffs from a NBA superstar from the RS to PS, which I think pushes things favorably in Nash's favor.
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Continuing on, I suppose I should provide some more data to back my Nash>Harden opinion, because looking solely at box-score numbers, this conclusion might still come as a suprise.
For one, Steve Nash was on a #1 offense for 9 years straight in his career, playing with Dirk and then becoming the #1 guy during this timespan. This gives validity to the idea that Nash truly was a special driving force and could mesh with other high end talent.
Furthermore, RPM and APM figures are very kind to Nash.
https://sites.google.com/site/rapmstats/rapm-by-playerAt this link not only does Nash have the top peak offensive peak per RAPM ahead of Peak Lebron, Peak Kobe, Peak Wade, etc., but he takes up multiple spots in the upper echelon.
According to Steve Ilardi's 04-09 APM, Steve Nash also has the highest offensive peak but quite the margin as well.
https://sites.google.com/site/rapmstats/04-09-6-year-apmFinally, I should bring up 19 year RAPM, which captured the downside of Nash's career yet still ranks him higher than Harden
While yes numbers aren't everything, the fact that Nash ends up so high in all these measures where we have a large samples size is telling. His playoff improvement and upped aggression in the second season gives more confidence in him such that if I had to pick between Harden or Nash to run my team, I would feel more certain what I am getting with Nash. Finally, a lot of people contribute Nash and Harden's success to D'Antoni's scheme but considering they have the same coach, I think we can be more confident that we are comparing their actual abilities and not just who is more well coached. Also I am focusing largely on offense as I consider both Harden and Nash as slight negatives on D, but neither being laughably worse than the other. I will say I think there is the chance we have not seen the best of Harden as a lot of offensive minded guys arguably hit their offensive peak in their 30s like Dirk, Bird, maybe even Lebron, etc. It will be interesting to revisit this convo in the future.
I then take Harden> Barkley
I think I take Harden but ironically it is not because of what he is most revered for which is scoring.
Rather, I think the huge gap Harden has as a playmaker is the real deciding factor and the fact that he might be less destructive to your defense since he is playing guard.
Harden's 3 yr PS ScoreVals from 14-15 to 18-19 are as follows: 1.1, 1,1.3, 0.7, and 0.8.
In comparison, Barkley's 3 yr PS ScoreVals from 88-93 are the following: 1.2, 1.8, 1.7, 1.8, 1.8, 1.6.
These are BIG and notable gaps between the 2 with Barkley clearly dwarfing Harden. To give you a frame of reference, the highest ScoreVal is about 3.3, so a 0.5 point is huge. However, it should be noted that Harden scored on much higher volume, but his efficiency left something to be desired in comparison to Barkley. Harden's ScoreVal was higher in the first years presented, because he was asked to do much less with OKC and therefore his volume was better. Harden likely could be more efficient if his offensive load was a bit less. Still Barkley's hyper-efficient scoring on around 24 pts/75 or so is nothing to take lightly.
However, Harden in 2020 just had his best PS run and he had a ScoreVal of 1.8, on a higher offensive load than anything Barkley ever had. Still, I think it is fair to say, that Barkley vs Harden might be a reasonable debate for scorers. Harden has higher volume, but Barkely makes up ground with efficiency, offensive rebounding, and maybe able to get more of his offense in a scalable way since he doesn't need the ball in his hands as much.
Now we take a look at their PlayVal.
Harden's 3 yr PS PlayVals from 14-15 to 18-19 are as follows: 0.9, 1, 1.3, 1.7, 1.7. Harden had a 1.6 PlayVal in the 2020 PS.
Barkley on the other hand: -0.8, -0.6, -0.5, -0.1, -0.1, 0.
Barkley's playmaking does not look great by this metric but he was indeed a strong creator for a big. Though his playmaking pales in comparison to great perimeter engines. The gap between Harden and Barkley as scorers is greater than the gap between the 2 as playmakers. For this reason, I believe that Harden is likely the better offensive player.
Defensively, Barkley was a negative as a bigman which is an even bigger red flag then being a negative as a guard, because ideally you want your bigs to add value through rim protection. Harden started out as a negative defender, but I no longer think of him as a negative on defense, and just about all defensive metrics would agree. Even if you wanted to pin Barkley as a neutral on D or so, relatively speaking, Harden is probably the better defender with respective to position.
88-93 Barkley (45 PS games played)
PER: 25.22
WS/48: .207
BPM: 7.24
Backpicks BPM: 6.28
15-20 Harden (73 PS games played)
PER: 24.85
WS/48: 0.186
BPM: 8.08
Backpicks BPM: 6.10
The box-score numbers are pretty darn close as you can see. BPM which favors scoring has Harden ahead, but the other numbers I am using favor Barkley. It is really up to you to determine if the numbers are overrating Barkley's efficiency or underselling Harden because he did more on a greater volume.
In the end, I chose Harden because it looks like he had the best single year peak according to these numbers and I do give him a little leeway running into the greatest 3 yr dynasty in NBA history in the middle of his prime. Harden's lows are lower, and I think possibly Barkley started off his prime better. But I think Harden's highs are better.