Common answer would be Tom Brady, but that's just incorrect.
Tom Brady career QBR: 95.7
Mystery undrafted QB's career QBR: 95.8
Said player: Tony Romo
Why is Brady considered the biggest NFL steal of all time? Any decent quality starting QB could've been plugged into Belichick's teams and won 3 Super Bowls. Not to mention Romo has had just about no team around him since he came into the league. No good coach or good GM like Brady had (arguably the best coach/GM in Belichick), never had a top defense, and never had a lot of elite weapons. Romo has 23 career 4th quarter comebacks, whereas Brady has 31 and Brady has been in the league for 6 more seasons (5 counting his injured season).
So, why isn't Tony Romo the elite QB that's consider GOAT and the biggest NFL steal? I guarantee you if Romo and Brady switched places, Romo would be 10x better than Brady.