Quite honestly you could visit anywhere in scotland and be close to some top class wild trout fishing. As a general rule though either go north of stirling or south of glasgow, that will narrow it down
One important point though, wherever you fish for brownies, you must keep on the move. They are territorial beasts and flogging the same bit of water all day is unlikely to produce the goods.
In my opinion you should aim to take in a mix of river and loch to get the full experience. Try one of the bigger rivers like the clyde, then try a wee hill burn the next day, trek to a wee remote hill loch and then go to one of the huge waters like loch leven. Variety is the spice of life, apparently.