The list by Sandy Nelson on Mike's main site is a good place to start IMO.
My "must haves" in the dry dept seem to be:
JT Olive 19/18 and 17/16
Wyatt DHE size 14 and 16 (sometimes SHE in 16 and 18 as Olive imit.)
Olive Paradun 18 and 16
CDC and Elk 12, 15 and 17
Balloon Caddis 17 and 15
BWO Spent Spinner 18 and 16
Black Gnat type thing 20, 18
If I had to narrow it down to just two for all season then JT Olive and Wyatt DHE as have caught all my fish since April. If only one then the DHE as it seems to basically represent both Olives and Caddis at different stages of their respective lifecycles.
I now seem to be pretty much dry fly these days but Waterhen Bloa, Snipe and Purple and Grouse/Woodcock and Orange are all good Spiders for me. For standard 'wets' Black Pennell and Bloody Butcher in the old days for the Sea Trout, March Brown for the Brownies.
Lindsay