There are lots of good leagues with lots of good players. In fact, every one of them has good players, and not all of the good players are in Cape Cod.
Often it is what the player makes of it. Most of all, the kid needs to play. But the Coastal Plains is outstanding, so is New England, so is the West Coast League and the Jayhawk League and the Northwoods League. It is tough and perhaps pointless to try to rank them.
Just go play and play your best.