Gerry, since you phrase the question in a whole different set of desires, wishes, cicumstances
--- this changes the answer quite a bit. First of all, it changes the stream to the South holston River if you are looking for a "16-24 in. brown trout". That might not be the best stream "at that time of the year" if you are just wanting a good day of fishing. To be able to catch a fish of the size that you mentioned, really changes the way that a guide would handle the way that he looks at the client. The average flyfisherman, off the street, would really have to have a lot of skills to just even be able to get a fish of that size to hit a fly. Handling a hooked fish of that size really gets into the highest of skill levels for any fisherman to have. Unless you're streamer fishing, you will probably be using a 6X tippet and that really narrows your chances of landing a fish that size down to about 5-10%. I don't know what Brett's skill levels are, but I do know that almost 95% of the clients that I take out need to have a considerable amount of work done on their casting and mending techniques. This is something that takes almost all day to get smoothed out, but when you get to where you can make 50-60ft. cast smoothly and mend properly afterwards, your chances of hooking a nice fish will go up a great deal. I think what I'm trying to relate is that if you have to work these other matters out while you're on your first trip, then asking to hook a real nice brown may take a few trips to get to that level. If he already has good casting and mending skills, he starts off with a great advantage. The guide only has to provide the right flies and show him where the fish are lying, or some that are feeding. The chances then go up many fold.
* This what most guides would do. Brett could tell the guide what he is wanting to do and what his skill levels probably are. The guide would either advise Brett to work on his casting and mending skills before they hit the water or he would immediately check Brett out when they get to a river and start working with the areas that need any improvement. That would probably happen in a wading scenario before the float starts if it's needed. He would also talk about insects and how to determine what the fish are feeding on "ie.dries or emergers" and how to tell the difference. He would show Brett the flies to use for each circumstance and how to present them. When this area is smoothed out , they should be ready to fish. This helps the client and also shows the guide any area that might need some work or attention.
* To get back to the original question, the time of the year will make a great difference on which river will provide the best fishing and I should note that the possibility of catching a large brown on the South Holston River will be much better a little later on in the summer when the Sulfurs start hatching in earnest. June or July are better months. The rivers that I mentioned first will be better in the earlier part of the year for just good all around fishing. I hope this answers some of your thoughts.
* *Hugh Hartsell---East Tn.