The key with receivers is release, catching and route running. Most big-time college receiver get let-down by the first as they aren't used to playing against good press coverage in the NFL. Receivers who might not go high in the draft but who have proved themselves against he press in college are usually the biggest steals, e.g. Steve Smith and Keenan Allen.
Watkins is a very good receiver but has yet to show he can get release from press coverage. Until he can show that then he is always going to be a but of a risk, much like Tavon Austin last year.
Marquise Lee has faced good press coverage in college, but he has really struggled against it. When he went man-to-man against Ipre-Olumu vs Oregon last year he looked nothing like an NFL 1st rounder. A big play receiver who will not develop into an every-down guy, similar to Ted Ginn in many ways.
Mike Evans is the best receiver in the class. At 6'5 225lbs he is a match-up nightmare. He has good hands and gets very good release, just watch the 'Bama game for evidence. Although he might not have the speed of Watkins or Lee he is a far more pro-ready WR, with just as much upside.
Summary: PLEASE GET EVANS!