I started following the Rams around 1968 and the 49ers have always been on my most hated teams list.
The Giants were the next team added to that list. The Giants were always on TV where I live and they were pretty bad, going 7-7, 6-8, 9-5, 4-10, 8-6, 2-11-1, 2-12, 5-9, 3-11, 5-9, 6-10, 6-10, & 4-12 while the Rams were busy winning *NINE* division titles. (The first division title the Giants won in my football lifetime was in 1986.) Watching the Giants to see if they would finish 3-11 or 4-10 while the Rams and 9ers played for the division title was a tough pill to swallow. I have since figured out that it wasn't the Giant's fault they were on TV, but as a kid I found it easy to blame the Giants.
The Cowboys and Vikings were added to the list in short order: the Rams made the playoffs 7 times between 1969 and 1978 and the Rams were eliminated by one or the other every time.
The most recent additions are Seattle and New England. (I don't think I need to explain ...)
AlbaNY_Ram