Crosby was dominating the QMJHL at 16 at a ridiculous level. He played for team Canada at 16 at the WJC. So, he had absolutely proven himself in juniors. I am saying if they were both eligible for the 2004 draft. Which would have been after Crosby's first full season in junior.
Tavares wasn't generational. He was never viewed as a better prospect than Crosby, and even after his ridiculous 16 year old season. I followed Tavares in the OHL, but I'll humor you and google, and look what comes up. Here is an archived version of a Bob McKenzie from that time. So, no, even at that point Tavares was not generational.
Stamkos before Tavares - Hockey Forum - Hockey Fan Forums - NHL Boards
This description (which took place to start the season right after Tavares 72 goal season) does not sound generational. If a majority of people view Stamkos as the better prospect, and some even viewed Doughty as one.
For a majority of Hockey's history, almost all the players have come from Canada. An inordinate amount of generational players are therefore likely to be Canadian. There have only been 3 generations (10 year spans) where the league has not been completely dominated by Canadians, and even during those eras, they have made up close to half the NHL.
I said there is an argument for Hasek and Jagr as generational, but neither of them was viewed as such as prospects. Ovi's in the discussion also. But, he wasn't the best prospect of his era, Crosby still was the better prospect in 2004 despite being a year away from his draft. Crosby isn't a top 5 player yet, and Ovi is still somewhere around 15. I also don't think Ovi would be taken over Crosby at anypoint during their career to build a team around except the years Crosby's career looked like it was in-flux.
I assume you are trying to make the case Dahlin is generational, which I don't agree with. Do you seriously think it is a reasonable expectation for him to be the clear best player in the NHL? Do you think he can win multiple Harts and/or Lindsey's or that he becomes a top 50 player of all-time? Those were all reasonable expectations for Lemieux, Lindros, Crosby and McDavid, and hasn't been for any other prospect to enter the league. Ovi wasn't a significantly better prospect than Thornton, Malkin, or Lecavalier.
And Bouchard just put up a PPG as a defender on London and its debatable if he's a top 10 pick.