When I think of biggest rebuilds, the one I always go to is Rich McKay and Tony Dungy with the Bucs. All those people ranting about Dungy in the Hall might contemplate what they were before he got there. Truly awful organization, seriously, look at the way they drafted in the 80s and 90s. Shocking. McKay becomes their GM in 1994, and in his second draft, he takes Warren Sapp and Derrick Brooks in the same round. Boom. That's how you get it done.
They build one heck of a defense. They were really bad before.
I don't know if the 2010s Lions count as a successful rebuild, but they're finally off the mat. Matt Millen left that franchise in a state of carnage like I've never seen. .242 winning % between 2001-08.
I don't know how Matt Millen lasted so long.