I'm more of a Lemieux guy.
Having said that, I certainly do believe that had things gone as well for Mario as they did for Wayne, he was capable of matching or exceeding Wayne's best years. When I say "had things gone as well", I mean generally perfect health playing 80 games per season, and maturing with his great linemates. One of Wayne's greatest strokes of luck (and obviously he can't be faulted for this) was coming into the league at nearly the same time as his best teammates - Messier, Anderson, Kurri, and Coffey. There was a great chemistry with these guys, that gave them the confidence to come back from 3 goal deficits in the 3rd period. If somehow, Lemieux had been able to skate with Francis, Jagr, Mullen, Stevens, and Tocchet (or Recchi) at a much younger and closer age, things could have been a lot better for him.
Now, when I first mentioned this before, The Panther did point out that it is also highly likely that Gretzky's best year was also curtailed by injury - in his 83-84 season Gretzky scored 205 in 74, which translates to 221 or 222 for an 80 game season. And considering he was injured, this could push the theoretical maximum even higher. And at these stratospheric levels each point becomes exponentially more difficult.
In a perfect world for both, I still do believe that Lemieux would have edged him out in a best season scenario , but I also do believe that Gretzky is the more consistent performer between the two, and would still retire with more career assists and points. He gave it his all every regular season, playoff, and international game. If the Oilers were up 6-2, he didn't take his foot off the gas. Ovechkin would have zero chance of catching a Lemieux that played 17-20 healthy seasons, in career goals.