I like a 5wt 8.5 ft because it's an all-around trout rod. The longer and faster are better for stillwater; shorter, slower and smaller for brookie streams.
6wt is light for pulling largemouth out of weeds, a touch heavy for stream work unless you're in the Deerfield or comparable sized river where a 7 is not out of the question.
That's been my experience over the years, granted not nearly as traveled as my steelhead or striper exploits.
I'm not much help, I know - but in my opinion a 4 or 5wt for all-round trout; and a 7 or 8wt for freshwater bass, pike, salmon is ideal, but the 6 will get you through it with most joy on big trout and small bass.