Without a doubt, it's Oncorhynchus mykiss.

But the question is whether this fish has spent any time in the ocean (a steelhead) or has spent it's entire existence in freshwater (a rainbow). To answer that question you have to presume that the fish takes on different traits depending on the environment in which it lives.

That's normally not difficult to do. As we all know, steelhead are usually large and silvery while rainbows are small and heavily spotted. But this fish has characteristics of both. Therefore, it could be either. I do not believe anyone can tell definitively whether the fish has spent any time in the ocean based on this one photo. The photo simply doesn't provide enough information. It's large size suggests an ocean existance while the coloration suggests considerable time in freshwater.

Flip a coin.....