The most dominant bears tend to do their fishing around Brooks Falls, visible from the Falls viewing platform. However the Riffles platform, situated about 100 yards downstream from the Falls platform, has some advantages. There is usually no time limit for occupancy, and you are allowed to use tripods here. It's also a great place to watch sows teaching their cubs how to fish, and subadults trying out their new skills.
You can view bears fishing in the Riffles via a NPS webcam: explore.org/livecams/brown-bears/brown-bear-salmon-cam-the-riffles