Interesting debate. When people say "generational player" does that mean he'll be the best player drafted by a particular team "in this generation"? or are we talking about the best 4-5 players in all of MLB within "a generation"?
Personally, I'm not sure I consider Harper to be a generational player - or even a no doubt HOFer at this point (although I more than likely he will get in). Harper certainly entered the league with a lot of fanfare and had a massive 2015. He's a great hitter - but I look at someone like Chase Utley and he may not even make the HOF, let alone be considered a generational talent. Utley had 5 straight seasons over 7.2 WAR. Harper has 1 in his career.
I never considered guys like Jeff Bagwell, Sheffield, Votto or Edgar Martinez to be generational players - yet they'll all probably end up with much higher career wRC+ numbers than Harper (and some will have more career WAR).
When I think of generational players, I think of Trout, Pujols, Griffey Jr., A-rod. And if we're talking just bats, guys like Frank Thomas and Manny. Even current players like Judge and Yordan are way ahead of Harper with the bat IMO. Great player, probably a future HOFer, but not generational IMO.