Depends whether you're talking about the NHL Draft, or just prospects in general....and the differences around NHL availability, USSR vs Russia....
If you start with Fetisov and Makarov, they were very big prospects and both were great players when they were teenagers.
You can even go back to Maltsev, Petrov, and Kharlamov, then Tretiak. All were big prospects.
After Fetisov and Makarov, Krutov was the next big one. Kasatonov just a bit less so.
In the mid '80s, Kamensky, then Tatarinov, and maybe Semak.
Mogilny and Fedorov were the same age, Mogilny was the bigger prospect at first (possible 1st overall in '87 Draft, if available), Fedorov became a very big prospect also, but a year or two later.
Then Bure, who would've easily been 1st overall in '89, if available (to play in the NHL).
After Bure, the next mega prospect was Kovalchuk, though there were lesser guys like Yashin, Tverdovsky, etc.
Ovechkin and Malkin were very big. I think I heard of them both when the were 14 or 15 years old.
Kucherov and Kaprizov were both huge scorers when they were teenagers. People have always underestimated them both, though, especially Kucherov. But they've never changed. Kaprizov is the best-scoring 19 year old in KHL history (ahead of Michkov).
Michkov and Demidov are great, they've been barred from participating in the WJC and other international tournaments, unfortunately.