As far as the expansion era (1967-present) goes, Mario Lemieux is the undisputed best No. 1 overall pick in NHL history, per Greg Wyshynski at ESPN, who unleashed his rankings earlier. And while several former top picks more than deserved consideration, including Sidney Crosby (2005), Lemieux may've been the league's most dominant player ever next to Wayne Gretzky.
Fans of other teams will have differing opinions on what I just said, but Lemieux changed the entire culture of Pittsburgh Penguins hockey. Yeah, they were ailing some when Sidney Crosby got there, but we don't know what would've become of the Pens if Lemieux didn't end up in the Steel City at the 1984 draft.