"He's got high-, high-end skill. He might be the most skilled player in the draft," said Toronto GM Dave Nonis.
"Electrifying," he added. "He has NHL speed, NHL hands and an NHL shot right now. It's whether or not the rest of his game can catch up."
The son of former NHLer Michael Nylander, William was named best forward at the 2014 Under-18 World Championship after leading all players in assists (10) and points (16) in seven games for Sweden.
Nylander, who was born in Calgary but lives in Stockholm, had one goal and six assists in 22 games with Sweden's Modo this season.
Just five foot nine and 169 pounds, the baby-faced Nylander looks younger than his years.
"He's grown a lot in the last 12 months ... He's taller, he's thicker," said Nonis. "There's a lot of work left there to do, for him.
"But in terms of foot speed, puck skills, ability to shoot the puck, he's got all those down right now. So right now it's going to be about hard work, getting stronger and prepared to play against North American men. He's already played in the men's league in Sweden, he's not a kid that's been playing junior. He's been playing in the (Swedish) elite League against some pretty highly skilled and strong players already, which bodes well for us."
Nylander is a free agent in Sweden, meaning he is free to come to North America.
"He'll definitely have a chance to make our team," said Nonis, while acknowledging it would be a "long shot" for him to crack the Leaf lineup as an 18-year-old.
"He'll be given that opportunity and if he's good enough to stick and play and contribute, then we would keep him. And if not, we'll decide at that point whether it's best to keep him over in North America or to have him go back to Sweden and play in the Elite League."
Quizzed about being ready for the NHL, Nylander said: "I'd say my goal is for next year, but if that doesn't happen, probably I'll go back to Sweden or play in the AHL."
Asked to describe his game, Nylander said: "I like to score goals and make plays and work hard on both ends of the ice."
Shaving looks like it won't be a problem for a while.
Nonis says the young Nylander makes spectacular plays.
"Those highlights happen on a regular occurrence," said Nonis. "So I think he'll be a very exciting player for our fans. He can move the puck, he's not just a guy who can carry it and snipe it. He's a good playmaker and at the under-18s, if you look at the points totals he threw up there, they were pretty impressive."
Nonis also liked the way the young Swede stood up to the Canadians when the two teams met at the tournament.
"I think he was tying to prove a point," he said.
Nonis said the Leafs were looking to move down in the draft had two players not been available by their pick. Nylander was one, he did not name the other.
Michael Nylander played for seven teams (Hartford, Calgary, Tampa Bay, Chicago, Washington, Boston and the Rangers) during his 920-game, 15-season NHL career.
William played briefly with his father last season when he was loaned to Rogle in Sweden's second division.