It's kind of splitting hairs between Mario and Ovi, but I'd take Lemieux.
Mario has the highest goals-per-game of any player in NHL history who played beyond the 30th birthday (although he only played about one
full season beyond his 30th birthday... but he did dress for 246 games from his 31st birthday onward).
While Ovi is clearly the best / most consistent high-volume goal scorer over a long period (at least, 2005-06 to 2022-23... and he mght not be done yet), I'm personally not that inclined towards longevity over a consistent prime, as long as that prime is several years. In Lemieux's case, during any period within his prime he was clearly a vastly superior offensive player to Ovechkin (as were several others in history), and he basically matched or surpassed Ovechkin's peak goal-scoring ability while playing as a center who put up league-leading assist totals.
So, we always run into this problem of trying to isolate "goal scoring" (or whatever skill) and extract it from overall (in this case) offensive ability. Like, Gretzky scored 700+ goals by age 30 while dominating the League every season in assists by the greatest margin ever. Is it really purposeful (or 'fair'... if such things are important) to rank Gretzky's goal scoring in isolation? The same applies to Lemieux and other playmakers who also scored a lot of goals.
I also think Lemieux had a superior skillset at a shooter / scorer than Ovi has/had.
Finally, can you please stop copying and pasting your blurb from the first post and sticking into every reply? Thank you.