I'd like to see at least one or two Euro scouts on record backing this statement up. It just has to be a somewhat reliable source as a reporter using anonymous scouts. Just because, I don't think this is a common opinion.
Articles come and go and I dont keep a tab on everything that mentions the kid but with a quick search here is one quote from a Shapiro article
“I don’t think it’s going to matter, he’s not going to care who’s watching, he’s just going to be (the Stars) second-best defender by December and the best one by the end of his second NHL season,” a European-based NHL scout said. “He’s the best prospect in Europe. (Rasmus) Dahlin will be more exciting, but Heiskanen is better overall in my view.”
From Dan Rosen of nhl.com on Heiskanen
"We're all going to be looking at Rasmus Dahlin as the best rookie defenceman probably, but it might be close"
I dont have time to scour the internet for any mention I have seen but that is one example of a scout who just straight up think Heiskanen is the better prospect. Also when I said Heiskanen > Dahlin I said that was in terms of skill right now. It doesnt take a quote from a scout to be able to go look at the past year and see that Heiskanen simply had a better season. That doesnt mean in the slightest that he will be the better player for the rest of the careers but right now based on play from the past year Im not sure how you can say Dahlin is the better player.