You have to establish what your metrics are. The fastest wideout? Biggest catch radius? Best hands? Best route technician? The best in this league all have different strengths so in order to figure out where a guy like Kupp would be the key is identifying what your priorities are.
My priorities are route running, intelligence, and hands. From those perspectives it's a very, very small list of wideouts who come out above him. In this offense Kupp is usually one of the top 2 or 3 players on the field vs anyone we play. He's elite IMO and I think that's why the Rams are going to ensure he stays in horns.