I'm not sure why we're obsessing on which player had the worst teammates. This has nothing to do with who the better player was.
It's pretty clear that when Gretzky was on the Oilers c. 1981-82 through 1987-88, he had great teammates (at least the top end), and when Lemieux was on the Pens' 1990-91 to 1995-96 (esp. '91-'93) he had great teammates. That's pretty much how it is when you win Cups.
In any case, just for fun, here's a Gretzky-focused break-down of his scoring vs. the #2 to #6 (i.e., next top five) scorers on his teams:
1981
Gretzky = 164
2-6 Avg. = 60.8
DIFF = 103
1982
Gretzky = 212
2-6 Avg. = 88.4
DIFF = 124
1983
Gretzky = 196
2-6 Avg. = 97
DIFF = 99
1984
Gretzky = 205
2-6 Avg. = 96.4
DIFF = 109 [pro-rates to 118 in a full season]
(Note: For this season, I actually calculated 'out' the games Gretzky missed.)
1985
Gretzky = 208
2-6 Avg. = 95.8
DIFF = 112
1986
Gretzky = 215
2-6 Avg. = 102.2
DIFF = 113
1987
Gretzky = 183
2-6 Avg. = 85.4
DIFF = 98
(I calculated 'out' the 1 game Gretzky missed.)
1988
Gretzky = 149
2-6 Avg. = 78.2
DIFF = 71 [pro-rates to 89 in a full season]
(I calculated 'out' the 16 games Gretzky missed, and I used Simpson's full-season total [not just Edmonton totals] in the teammates' average.)
1989
Gretzky = 168
2-6 Avg. = 88.6
DIFF = 79 [pro-rates to 81 in a full season]
(I calculated 'out' the 2 games Gretzky missed.)
1990
Gretzky = 142
2-6 Avg. = 75.2
DIFF = 67 [pro-rates to 73 in a full season]
(I calculated 'out' the 7 games Gretzky missed, and I used Nicholl's full-season total [not just L.A. totals] in the teammates' average.)
1991
Gretzky = 168
2-6 Avg. = 71.8
DIFF = 96 [pro-rates to 98 in a full season]
(I calculated 'out' the 2 games Gretzky missed.)
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Overall, then -- and using the "pro-rated" averages to account for missed games -- Gretzky between 1980-81 and 1990-91 was outscoring his 2nd-to-6th-highest scoring teammates by an average of exactly 100 points per 80 games.
Here's a closer break-down:
Gretzky's vs. average of scorers #2-#6
1980-81 to 1990-91 (total 11 years)
100
1981-82 to 1985-86 (Gretzky's point-scoring peaks)
111
1988-89 to 1990-91 (first three L.A. seasons)
84
What's significant, I think, in all this is that Gretzky's domination of team-scoring does NOT peak as his teammates are weaker -- in fact, it's the exact opposite. The more his top teammates were scoring, the more Gretzky was scoring. Or, it could be that the better Gretzky was producing, the more his teammates were also scoring, driving everyone's totals up.
There are some exceptions here, like 1981-82 where top teammates were good but hadn't peaked yet, and Gretzky was driving offense individually like never before (or after, maybe), so he ended up 124 points above the #2-#6 average, which is crazy. Likewise, that last season of his prime, 1990-91, where his L.A. teammates generally weren't scoring in huge amounts but he ended up dominating them more than usual (adding further fuel to the argument that 1990-91 was Gretzky's best post-Edmonton season).
Anyway, how does all this compare with Mario Lemieux? I have no idea because it's too difficult for me to calculate Lemieux's / teammates' numbers with all of Mario's missed games. We can do a quick look at a few seasons where he played nearly all the games:
1987-88
Lemieux = 168
2-6 Avg. = 61.8
DIFF = 106
1988-89
Lemieux = 199
2-6 Avg. = 85.8
DIFF = 113
1996-97
Lemieux = 122
2-6 Avg. = 69.8
DIFF = 52
The interesting season to break-down would be 1992-93, because it's the most stacked Pens' team ever and the only season they were 1st overall. So, it's the closest approximation of a Gretzky-Oilers season in 1984 to 1986. I could do the math on that, but frankly it's too much work, so maybe someone else can....