Yeah Salah is a World Class player, or at the very least has been a World Class player. No argument from me there. Although Son and Mane are not considered to be World Class players by you, I'm sure we can agree that they have been two of the top performing forward players in the league over the past 5 years or so.
Despite being good/very good players at various points, Sterling and Vardy are players who I wouldn't consider to be World Class. Sterling had the benefit of playing for the best team in the country and Vardy was a bonefide centre forward. According to Transfermarkt, Rashford has 12 goals and 3 assists in 1,741 minutes in the Champions League. This equates to just over 19 games. Not too bad considering that we have hardly been sparkling in recent years.
As for the 'best in the world' and 'World Class' labels that have been bandied around, I would probably agree with you. For now. However, it isn't that much of a surprise to see Rashford performing at his current level because we've seen it before. Athough perhaps not quite as good as this. It's just a question of longevity because he needs to continue at this level year after year. We look to be on the right track now as a team and Rashford is finally playing under a talented and progressive coach on the up. Time will tell.
Just to add, we all have our own definitions of what constitutes a World Class or top class player. My own unusual and childish theory would be to pretend that you are constructing a Fantasy Football Team. Write a list of the top 10 or 20 left sided forwards in the world, down on a piece of paper. Seperate them, tear them off individually, scrunch them up and put them into a hat. You've just picked out...'Marcus Rashford'. How are you feeling about that? Personally, if I'm picking out anyone other than Mbappe, I'm a bit disappointed. However, Mbappe is in a league of his own, so who would you hope for as your next best option? I couldn't think of too many I'd want over Rashford. You could debate Vinicius Jnr or one or two others perhaps, but after Mbappe, I'd be pretty content with having Rashford in my team.