The CHL, taken on the whole, is far and away the best junior league in the world. Keep in mind that the J20, it's almost exclusively Swedish kids. Same for the respective leagues in Finland and Russia. The CHL has top talent from all over the world and the majority of the top talent from the two countries that produce overwhelmingly the most NHL players.
There's also the fact that the teams in the other leagues' junior divisions are associated with parent professional clubs, so if you have a dominant under-20 player they'll often be playing not in juniors but with the parent club (or moving back and forth). That's much more rare with CHL guys since they have to be able to either jump straight into the best league in the world, the NHL, or finish out their CHL careers (obviously there are exceptions based on where a guy was drafted from, but speaking generally).
The J20 is a very good league. Good instruction, high level of play. But it's not close to the CHL, IMO. Five years ago I'd say it was better than the USHL, but the USHL has been making major progress with more and more players from NA and all over the world choosing to go the NCAA route.