It really depends on where I'm going to fish.
For little brook trout creeks, I will usually use a 7' 4 wt bamboo or a 7'9" TFO Finesse.
There are a couple of medium-sized rivers in Connecticut where a medium-to-medium-fast 8 1/2' or 9' 5 weight is just the ticket.
There are a couple of big brawny rivers in northern Vermont and NH, with tricky currents and deep holes, where I never know whether I'll be fishing #18s on the surface, or deep streamers with a lead head, or weighted nymphs with indicators on long casts, or soft hackles on a down-and-across swing. That's when I really need my pretty fast 9' 6 weight that can do a little of everything. I know people out West who like a fast 4 weight for the same sorts of situations, but I think that's like trying to swim with one hand tied behind your back.
Fishing for bass in lakes with big deer hair bugs or weighted streamers on a sink tip usually finds me with a medium-action 9' 8 weight.
I've taken a few trips to Quebec and Labrador for Atlantic salmon, where the best rod seems to be a medium-fast 9' to 10' 8 or 9 weight, or 8 overloaded with a 9 line. On my last trip I also took a 15' TFO 8/9 spey rod, and caught a few fish with it that I wouldn't have been able to reach with a 9 footer, but that's a pretty infrequent situation.
Fishing the New England coast for striped bass and bluefish seems best with a fast 9' or 10' 9 weight, to help with the long casts, wind and bulky flies.
If I had to narrow it down to only one that could cover the widest variety of situations adequately it would probably be an 8 1/2' to 9' 5 or 6 weight for trout, panfish, and light-duty bass fishing. (Which is just about exactly the conventional advice for beginners that you can find almost anywhere.)
