Jim, Having a 4, 5, and 6 weight I can say that if I were restricted to one all purpose trout rod I'd go for the five weight. It splits the difference between the 4's presentation and the 6's larger fish/wind capabilities. I'd get an 8'6" rod, but thats because I fish brushy streams more than stillwaters.
The line I fish 95% of the time is a weight forward floater. I am not a fan of intermediate lines for freshwater so the other 5 % of the time I use either a sink tip or a full sink line. If I fished more stillwaters I would probably use the sinking line more. Also, there is a much better selection of sinking lines for 5 or 6 weights than 4 weights.