It gets tough for the first few after Allen because:
1) Some of these guys had careers outside Buffalo (e.g. Fitz, Bledsoe, Anderson)
2) A few of them had one good season in Buffalo but then went bad (Bledsoe, Fitz, Flutie, even Tyrod)
3) Others flashed at times but could not perform consistently
And then there are some that were just plain bad.
My list:
Allen
...
Bledsoe (his good year was very good, then faded hard)
Flutie (misused, good in 98/99, also faded hard in 2000, but was electric before then)
Fitz (maybe the same average result as an average QB but has a high variability that gives hope. Did bad after contract).
Taylor (Better results for the team but faded during his time in Buff)
Orton (Gave us some good games but ultimately nothing special)
Beyond that it is a mess. Next tier - guys who showed promise but failed:
Rob Johnson (He "flashed" at times but his sack rate was just unacceptable by any measure)
Trent Edwards (Started out good then went bad, whether is was a concussion or bad coaching who knows)
Next tier: Guys who were not good. Ranking is very hard.
JP Losman
EJ Manuel
Kelly Holcomb
Alex Van Pelt
Todd Collins
The rest... bad and short lived:
Matt Barkley (had one good game)
Thad Lewis
Derek Anderson (not good here but had legit career)
Jeff Tuel
Brian Brohm
Nathan Peterman (the worst)
No rating:
Matt Cassel (started only on a technicality, much better career than others listed, did less harm than Peterman)