I'm with Arky here.
My three most hated Rams-rivals are San Francisco, followed by the Niners but ... most of all, the 49ers!.
Now I consider "hate" to be a very strong word; so, to be clear, I hate the San Francisco 49ers.
It's not healthy; and a lot of it has to do with jealousy. I am not proud but it's the truth.
I go back to 1970. Also disliked Dallas and Minnesota but that had much to do with their playoff-success against the Rams. The Cowboys beat the Rams in 1973, 1975, 1978 and 1980. Dallas beat the Rams in the NFC Championship in '75 and '78 which was very disappointing but the Cowboys dominated both games. However, the Rams have also enjoyed playoff success against Dallas, with victories in 1976, 1979, 1983 (all in Dallas!), 1985 and 2018. Those were all sweet victories that helped to balanced-out the defeats.
Now, to this day, outside the of the Super Bowl losses, the most-painful defeats in my Rams' fandom were the 1974 and 1976 NFC Championship Games in Minnesota. Always thought the Rams were the better team but lost extremely close games.