4. Los Angeles Rams
The Rams have had an interesting history. They started out in Cleveland all the way back in 1937 and remained there until 1946. Since then they moved from Los Angeles to St. Louis and back again to LA. The Steelers' first-ever contest against the Rams came way back in 1938.
If we are counting all of their contests prior to the NFL merger, the Steelers were about as abysmal as a football team could possibly be against one particular opponent. This continued even a decade after the merger. From 1938 to 1980, Pittsburgh had a disgusting record of 1-12-2 when facing the Rams.
Fortunately, the tides have since shifted and the Steelers have owned the Rams over most of the past five decades. It's rare that you see a team pull a complete 180 like this, but Pittsburgh has a 9-3 record against the Rams since 1981.
Still, the insanely poor head-to-head start gives Pittsburgh an overall record of just 10-15-2 when facing the Rams. This puts their all-time win percentage at just 40.7 percent. This is an uncommon opponent, so a few more wins in a row from the Steelers could change this percentage drastically.