I'd probably say Edmonton or Toronto or Buffalo are the best. The Flyers certainly are up there, but I don't think I'd put them as the best. Probably #3 or #4.
Flyers have ATM 21 and under the Neck, Provolone, Lindblom, and Patrick in the NHL (and Frost and Myers on the cusp). Then from 22-25 they have Sanheim, Ghost, Hagg, Coots, Laughton. That's pretty impressive, but you have a team like Edmonton with McDavid, Draisaitl, Yamamoto, RNH, Puljujaarvi, and Larson (plus Klefbom and Nurse). Or Buffalo with Eichel, Mittelstadt, Dahlin, Reinhart, Risto (I know, I know, he sucks), and Thompson. Then you have Toronto, whom out of these three I am hoping fails the most, with Matthews, Marner, Nylander, Kappanen, and Rielly, plus some 25-30ish players that will make an impact in the immediate future but maybe ten years from now won't like JT and Kadri. These teams (with the exception of probably the Leafs) some prospects that are farther off that may or may not have the impact that we think. But if I'm guessing which team will have the most success over the next ten years it would be these four. I know that these teams have flaws (some of which includes management and coaches), but if we are just talking about the rosters or organizational depth, this is where I'm looking. The one thing that the Flyers have over these teams is Carter Hart, but he's all potential right now so I'm not hanging my hat on him. If he hits (or Sandstrom or Tomek or Stolie or Fedotov or anyone in the system) that probably puts the Flyers much closer.