What's the first thing you think of when I say: Magnum?
above said question.
For me: Colt Python. Reason being, toy guns were plentiful when I was a boy. Particularly cap guns that were revolvers that popped those red plastic moonclips in a steel cylinder. The guns actions usually broke within 2 days of buying it. Usually I had a snub nose .38 or a cowboy gun....but one day in the toystore I got one labeled "Magnum" Not a snub nose barrel, but a longer one with a vented rib and a full underlug under the barrel. Daddy came home from work and I showed him my new gun. He took it in his hand and said "that's a magnum" Something really unique and powerful looking about it.
Having said that, if I think a little more historically I think S&W N-frame .357 Magnum with a 6 inch barrel. A long barrel mammoth revolver. The original frame built for the first magnum gun in .357 in 1935.