In 1962, Cleveland Republic Steel suggested to the Steelers that they adopt the Steelmark logo as their official logo. The Rooneys like the idea, and soon, the the Steelmark logo that belonged to the American Iron and Steel Institute also belonged to the Steelers.
They wanted to put it on their helmets, but the fashion-conservative Rooney family, not wanting to to do anything too flashy or daring (understandably, since we're talking about Pittsburgh here, where they've only just now started to believe that people without moustaches could be decent, trustworthy people), experimented first with just putting the logo on one side.
That way, if the people of Pittsburgh turned out to not be ready for such a fashion-forward move, they could just, I don't know ... move to other side of the stadium and look at the blank side.
Myself, I rather like the three touching hypocycloids. They make use of the three primary colors, the very bedrocks of our visual abilities, and there's meaning behind each hypocycloid, too.
When the Steelmark logo was created, U.S. Steel attached the following meaning to it: Steel lightens your work, brightens your leisure and widens your world. The logo was used as part of a major marketing campaign to educate consumers about how important steel is in our daily lives. The Steelmark logo was used in print, radio and television ads as well as on labels for all steel products, from steel tanks to tricycles to filing cabinets.
In the 1960s, U.S. Steel turned over the Steelmark program to the AISI, where it came to represent the steel industry as a whole. During the 1970s, the logo's meaning was extended to include the three materials used to produce steel: yellow for coal, orange for ore and blue for steel scrap. In the late 1980s, when the AISI founded the Steel Recycling Institute (SRI), the logo took on a new life reminiscent of its 1950s meaning.
Coal isn't yellow, but whatever. Let's not split hairs.
Anyway, the fans didn't object, and the Steelers followed the adoption of the logo with two other tweaks: They changed their helmets from gold (you saw these when the Steelers wore throwbacks this past year) to black, and they got the American Iron and Steel Institute to let them put "Steelers" in the logo, as opposed to just "Steel." That happened in 1963, and since then, things have remained unchanged.
You combine that kind of history and consistency with the overwhelming amount of quirk in the Steelers helmet, and I do believe you've got an undisputed champion here. It's quirk city on the side of Ben Roethlisberger's head. Observe:
• It's the only professional helmet to use a logo that was once a corporate logo before it became a football team logo (other than the short-lived and now defunct Miami Hooters of the Arena Football league).
• It is the only helmet in the NFL to have a logo on one side, but not the other.
• There are two colors present in the helmet logo that exist nowhere else in the Steelers uniform: red and blue.
It's extremely unique, it's got an interesting backstory, and obviously, there's a lot of winning tradition behind it. It succeeds on every front, and is something that's kind of evolved organically out of the town itself to become a powerful and iconic sports logo.
Thankfully for the local football team, Pittsburgh was, in the 60s, a city known for the hard, durable, solid steel it produced. If Pittsburgh had been a city known for cotton, latex, or their burlesque shoes, franchise history could've been totally different.
The Honorable Mentions
#11: Buffalo Bills, '65 - 73
#10: Edmonton Eskimos, '70 - Present
#9: San Diego Chargers, '61 - '73
#8: Iowa Barnstormers, '95 - Present
#7: Cincinnati Bengals, '81 - Present
#6: Philadelphia Stars, '83 - '85
#5: Dallas Cowboys, '77 - 'Present
#4: Houston Oilers, '75 - '80
#3: Tie: Green Bay Packers and Indianapolis Colts
#2: Tampa Bay Buccaneers, '76 - '96