Sammy -
After 13 years of the L Sac being my office, I'd have to agree with Ed K on this one.
The BHFBPT (bead head flash back pheasant tail) has to be one of the most consistant flies that will catch fish ALL YEAR.
Many good answers here, but....
--Beads aren't flies....

and the fish don't eat them all year long.
--Caddis pupa imitations rarely work 'year round'. The fish eat caddis during certain times of year. I do love the poopah and the bird nest tho! (we're coming into that time soon!!)
The mayflies are always on the menu. And it's a fly tied with fur and feather.
As a matter of fact, I think if I had to fish only one nymph for the rest of my life, it would be the BHFBPT.
sd