Harry Kane is unquestionably World Class and even as a massive United and Lukaku fan, I have to admit Kane is the better striker, he's more clinical and has a far better touch. As far as out and out No 9's go if we include Christiano as one, the current Rankings would probably look like something like this ;
1. Ronaldo
2. Lewandowski
3. Kane
4. Aguero
5. Cavani
6. Lukaku
7. D Costa
8. Suarez
9. Aubameyang
10. Higuain or Benzema
Of course it's completely subjective with other players like Lacazette, Mbappe, Rashford, Tino Werner, Mandzukic, Icardi, Falcao, Moratta, all being great strikers but are they focal point of the team with most, either playing wide to accommodate others in the starting line up or not having the consistency of the above group.
Ps I've probably missed someone out but most will agree Kane is in top 3 to 5 in the World?