I nearly said Anchorage, because I got a lot of Anti-Boat Emojis, but memes aside, the biggest rival of the Kelowna Knights probably are the Vancouver Whalers, judging by location alone. We also share green in our colour scheme while green/gold is far superior to green/blue.
As far as I remember Vancouver had not much going for them the last seasons, dating back to my Nevada time at least in S65. That said Kelowna really didn't do much better, but we now have poached a new GM and got an active LR with decent earners. We might be able to make a splash in the playoffs before becoming legitimate contenders in 2/3 seasons probably.
Can't say how much of that also rings true for Vancouver as I do not know much of their current status apart from knowing Seany from my stint as HO intern. I wish them luck though...I mean booo, stinky rival!