A 9ft 5wt is a pretty standard, do all trout rod. If you're fishing small streams only maybe something shorter like a 7.5-8ft 3-4wt.
I have 3 rods.
1. 7ft 3wt which I use for small native brookie streams.
2. 8.5' 5wt which is my all purpose rod that I do probably 70% of my fishing with.
3. 10ft 6wt that I use on larger streams/rivers or for throwing streamers.
Nighttime rod - 9' 8wt (for both tossing bigger flies, and for the bigger brutes that tend to show up at night).
But if you had to pick one rod, the 8'6" - 9' 5wt would cover most situations, and you could shoehorn it into situations where another rod might work better, but a rod in that length and weight would still be serviceable.