Well since you will be using it on the Susquehanna......an #8 will be a good choice.
I use a a St. Croix 9' #8 "Legand Ultra" for the Susquehanna down my way and it works great!
You really don't need an #8 for FIGHTING most of the fish (occaisionally yes)......You need it to beat the wind, and as Lil' said, to throw the size flies you'll need.
If you must have one line. make it a floating Bass taper.......but one of those multi-tip lines from Sci Anglers or the like might be ideal......Floating tip, Clear intermediate, SLow and Fast sinking all in one package. You can change tips to match conditions.
On calm days a #7 or even a #6 or #5 will work for dry fly fishing......such as when the Epeorus (White Flies) are hatching and you are flicking around big White Wulffs.
An #8 gets a bit tiring after a while with this type of fishing.....but you'll definitely appreciate it when you need to boom casts into the wind and handle all that line on big water like the Susquehanna.
It will also handle the occaisonal carp or big channel cat you come across feeding.......a 20lb carp on a dry fly and an #8 rod is something you'll not soon forget!!<img src=icon_smile_big.gif border=0 align=middle>
If you can only get ONE rod......make it an #8
Check out the St. Croix "Legend Ultra".....IMO it is the best rod on the market in it's price class. Followed by the Loomis GL3.
A nice large arbor reel such as a Ross "Canyon", Bauer "MacKenzie", Loop, Orvis,Redington or whatever brand is a big help with it's faster line pick up capabilities.
I use a Teton "Tioga" LA and it's been pretty good for me so far.........
By the way......an #8 outfit will also allow you to chase Great Lakes Salmon,Steelhead, and Browns around if you ever get the desire.