In my BMW, I'd have to say 2nd - when you shift the eta at redline in first, the ratios are so perfectly chosen that it places the engine right in the peak of its torque production and it really gets up and goes.
In my Camaro, I'd have to say 1st. Launching it hard from a stop and winding 350 cubic inches of carbureted V-8 out to 5500 rpm is like heaven, especially once I hit the cam's onslaught of torque at about 2200 rpm. Although 2nd is pretty awesome too, since the car pulls hard all the way to redline, acclerating hard the entire way - by the time you reach redline in 2nd, the car is going about 80 mph.