I'll second those who advocate for Stabler. I won a lot of close games late with him (before my cursed losing streak), and have found that he's really good in clutch situations. He can throw inside the pocket and on the run, and can scoot a little if you need him to.
From the bronze tier, I think Kosar is a pretty good pickup if you're looking for someone to execute rather than someone to carry your team.
From the gold tier, I haven't played with them all, but I think Elway is a really good choice. He's got the cadence thing, which can get you a crucial first down here and ther, plus he does some insane things in clutch moments, and he can avoid the rush. Of course, if you have Marino, there's no need to because of the quick release
I'll also second the earlier poster on the value of Rice. I don't have him on my team, but I've played against him a couple of times, and he's done things that even the real Jerry Rice can't do. Your QB's value goes up dramatically when you've got someone like that on your squad.