There are a couple of locations for each store---which are the best ones for a teen? I would prefer to go to the ones near Rockefeller Center on 5th Avenue

The Lego store in Rockefeller Center (620 5th AV @ W50th St) is considered to have the best layout and selection. .https://www.yelp.com/biz/the-lego-store-new-york

The Niketown store at 6 E57th St (Trump Tower) closed in March 2018. The new store - 650 5th Av @ W52nd St is not expected to open until early 2019 (according to newspaper reports). At present the Nike SoHo store at 529 Broadway (between Prince St and Houston St) is the nearest large store. (N or R train to Prince St, or B, D, F and M trains to Broadway-Lafayette St) https://www.nike.com/us/en_us/retail/en/nike-soho

Is the Soho store worth seeing, like the one that closed?

I have not been in that store but have walked by it. It is not as big as the one that was on E57th. I would have to say that from a visual standpoint it is just a regular store rather than the one that E57th was.