I do too many different types of fishing to simply have one rod.
I use...
7ft #4 for small stream stuff
9ft #4 or #5 for river trout & grayling and for fishing proper flies on still waters, currently using a Helios 904 midflex and a 905 tipflex.
9ft6" #6 for sea trout, singlehanded salmon, chucking lures on stillwaters, big streamers on rivers, jack pike in small canals, and light saltwater, using a Helios 966 tipflex at the moment.
9ft #9 for pike and heavier saltwater work currently, rod of choice is a TLS Power matrix, heavy but more or less unbreakable.
14ft #9 for most salmon fishing where a DH rod is required and a 15ft #10 for throwing full sinking lines or on big rivers.
For casting against the tape I still like my 590 TCR.
If I had to keep only one, it would be the Helios 966 tipflex. With a good choice of lines I would be happy to use it for more or less anything.
Cheers
Ben