Cowboys and it's not even close.
There are of course hardcore cowboys fans, but they have the largest bandwagon fanbase that has no clue about anything.
Perfect example was I saw something on another message board where a guy said "David Carr is my hero!" after Brandon Carr picked off Big Ben and sealed that game basically.
An hour later "David Carr" was trending on twitter. I'm sure the former Houston QB is loving the attention
They have a lot of fans who do it just for attention, with the most existing in the DC MD VA areas. We call them NBD (Never Been to Dallas) Cowboys fans.
Steelers are probably 2nd, but they also have way more true fans than the Cowboys.
Edit: I do hear quite a bit though that RGIII has a lot of these bandwagon cowboys "fans" in Texas and in DMV area torn and jumping off the bandwagon. Guess it's always about how to best get attention with these types.