There are a number of great wide receivers that have played in the NFL over the years. Don Hutson, Lance Alworth, Paul Warfield, Steve Largent, Jerry Rice, and Terrell Owens are all great wide receivers. They were all drafted into the NFL coming out of college too.
The best wide receiver the Denver Broncos have ever had was Rod Smith. Smith played collegiately at Missouri Southern State and then was not drafted by any team when he came out of school in 1994. He was first signed by the New England Patriots, but was released a short time later, and then he was signed by the Broncos.
Over the course of his career, he would catch 849 passes for 11,389 yards, and 68 touchdowns. Those are all the best totals in those categories for any receiver ever to go undrafted. He is also the only undrafted player to ever go past the 10,000 yard career receiving yards list.
Rod Smith proved the college scouts wrong by eventually putting a career together that was so good it would see him inducted into both the Division II Hall of Fame and the College Football Hall of Fame. He thin proved every team in the NFL wrong by putting together one of the great wide receiver careers ever even though they all passed over him multiple times in the draft. Rod Smith is definitely in a league by himself.