i'd say Jeff Hostetler was the best back up ever, stepped into the last game of the 1990 season after Phil Simms got hurt after barely playing for 6 years and killed it in the playoffs and won the Super Bowl
plus I was always a fan especially when he came to the Raiders and gave us a playoff win over the Donkeys
But Eli manning could win 1 more and he might be arguably better than Brady. If Eli wins 2 more Superbowls you would have to discuss him as an option of best QB.