I'm going to guess that you mean the biggest freshwater fish that one can easily purchase. If that's the case, then I'd say that an RTC x TSN hybrid is most likely your best bet for a commonly available fish that will grow fast and has the tendency to reach a very large adult size.
Sure, there are plenty of other fishes that can get huge as well, but the vast majority of those are not easily acquired and/or grow extremely slowly. Most of those fishes also tend to never regularly reach the maximum lengths that are often brought in discussion by those who are grossly misinformed, and end up reaching a much smaller size than the known maximum.