9' 5wt if just fishing CO in general, and a 4wt would be a close second (disclaimer: I primarily fish a 4 and 3 in CO but bump it up a notch in WY and MT). Action is entirely dependent on the angler: one likes a really fast rod, another prefers a more parabolic taper. Go to a shop that stocks a LOT of rods from various brands and cast every single rod you can get your hands on. This is NOT a situation in which the most costly brand is right for every person: your casting style might do better with a rod that costs half what the most expensive rod in the shop does.

Do not be swayed by brand prestige, pick the rod that casts best for you.