Top Dynasty UDFAs in History

*** This is meant for UDFAs in dynasty Rookie Drafts, I know Pacheco wasn't a UDFA in the real draft ***

Alright so a leaguemate and I were talking about how the best case for a UDFA in dynasty is likely a RB that overtakes a vet to be serviceable, but not necessarily a stud (my example would be Tyrone Tracy taking over for Singletary).

We just finished our first year of dynasty so we were curious for historical precedent. Like was Pacheco even a UDFA for drafts done in June? What were some of the best you've seen?

For sake of argument and guidelines let's use a 12 team league with 4 rounds in the rookie draft. 1QB or SF.