Broncos always had a solid fan base even before the Elway era. Current bandwagon now has to be seattle. After the media was forced to take the lombardi trophy away from jimmy baby-screamer harbaugh for the 4th time this season, they've all jumped to the seattle trojans (or is it the usc seahawks?).
As for fair weather, I've lived in a lot of places and I have to say its the giants. Non-giants fans here in Jersey have a saying: you won't find a giants fan anywhere when they're losing. Its almost uncanny; they seem to have a 6th sense before the game even begins and they are nowhere to be found on the days the giants play bad. On days the giants come from behind and win, their fans are in hiding until that magic moment when eli hits cruz for a score. Then all of a sudden there are about 35 of them screaming and yelling.