There is a strong case to be made that Derek Loville is the most successful career backup of all-time. Loville was never formally drafted by an NFL team following a fairly respectable college career at Oregon, but he did join the Seattle Seahawks in 1990. He would rotate in every now and then and performed well enough on occasion to justify a team picking him up to fill a roster spot. Those roster fill-ins led to Loville winning one championship with the 49ers and two with the Denver Broncos.
Loville enjoyed a quiet retirement until earlier this year when he was indicted as part an investigation involving a drug trafficking ring. Loville was apparently also involved in the ring’s money laundering and illegal gambling and apparently would even intimidate their enemies by threatening to behead them.