It depends on where I'm fishing and so it's hard to come up with just ONE go to fly but some of the flies I fish most often are as follows:

ADAMS
ELK HAIR CADDIS
BWO
Dave's HOPPER
LIGHT CAHILL

and for nymphs it's:

GRHE
PRINCE

I may be soon adding the PATRIOT dry fly after I've fished it a little more.

Scott