I don't know about you guys, but I tend to look at overall history rather than recent history when thinking about the worst franchise in professional sports. I think the question should be broken down into who is the worst in recent times and who is the worst over a prolonged period. If you're looking short term, then shit, the Pirates are the absolute worst. But if you look a little further back, the Pirates are a decent enough franchise.
I know that things haven't been good for the Pittsburgh Pirates in recent years (18 and counting) but they wouldn't be on this list if they had just managed one or two winning records during that time. They've won five championships and they have 12 players in the Hall of Fame, tied for sixth. Their last two titles, in 1960 and 1979, are noted for being among the greatest World Series' of all-time, both going to seven games and featuring dramatic finishes. Now, I'm pretty sure nobody wants to be the fan of the team who sets the record for continued futility, but I'll bet anything that Cubs fans will take 18 straight years of suckage in exchange for one of the titles the Pirates have won in the past 102 years (funny enough, the Pirates won their first title in 1909, the year after the last Cubs championship). I also don't like the idea of the Raiders being on the list based on their history as well.
However, like I said, if you're looking short term, then the Pirates and the Raiders are the pits (what's wrong with the sea-faring criminals these days?) and not just for their shitty play on the field either. Both teams have been marred by some terrible front office moves. The Pirates don't want to spend money, while the Raiders waste truckloads of loot on bad players. The Raiders are looking okay so far this year, but the Pirates will probably go another couple years before they have a realistic shot of getting a winning record (Pedro Alvarez better be the man!)
For my worst picks, based on the long term, are the Arizona Cardinals, the Los Angeles Clippers, the Detroit Lions and the Phoenix Coyotes/Winnipeg Jets. These four franchises combined have managed just six league championships (two by the Cardinals and four by the Lions when it wasn't even the Super Bowl yet, just the NFL Championship). Despite being in the NFL since 1920, the Cardinals have only made the postseason just eight times!! They just made it to their first Super Bowl two years after years of failures and cutting corners by the Bidwells. The Lions have yet to make the Super Bowl. As for the Clippers and Coyotes/Jets, they've both failed to even win a conference title during their respective existences. In fact, they've combined to win just three division titles, all three by the Jets in the early 70s!
So yeah, I suppose your opinion on this will vary depending on how you look at it, but those are my picks based on long term futility. I was also thinking of incorporating other factors such as stadium quality and attendance, but this post was already too damn long.