When I joined this league in Season 49, the biggest rival for our franchise was the Anaheim Outlaws. Story time! You see, the Lethbridge Lions used to be an SMJHL franchise in the olden days (think Season 5-10ish). The franchise was revived, at the same time as the Outlaws were created, in Season 45, to deal with the influx of new S46 players coming in - which was a very astute decision, as the Outlaws ended up winning the Cup in S48 with mostly S46 players leading the charge.
That Cup win soured us in Lethbridge, as we had been left in shambles by bad management, and a seemingly empty Locker Room. jnh and SDCore, with the help of Carpy and I, looked to reinvigorate the franchise - and that's where the decision to rebrand to the Newfoundland Berserkers came. The transition of the Halifax Raiders to the Maine Timber happened at around the same time, so the timing was perfect for us.
Since the transition to Newfoundland, we have found ourselves an even greater rival, one that stopped us from fully establishing dominance on the Eastern Conference : the Carolina Kraken. Not only did they move out of beautiful Montréal (a shame, really), but they were always in the thick of things to compete for the Four-Star Cup - and eliminated us in back-to-back seasons, en route to their championships. When we finally beat them (and won the Four-Star Cup!) in Season 55, it was like having a 400-pound gorilla off our backs. The rivalry isn't as heated as it used to be, but in my opinion, all it'll take is one small playoff series for the hatred between those teams to reignite.