It all depends on what type of fly fishing you are doing, river or lake? And also how large the venue is, how many people fishing it frequently and if there are any sizeable fish in it. As you can see, there are a lot of factors to take into account.
River fishing – Nymphs
Pheasant Tail Nymph (PTN) – with a gold bead for faster waters and mainly to get it down to the fish, start with a size 14 and go up to a 10 if required.
Hare’s Ear Nymph – again gold bead if required and start on a size 14.
Diawl Bach – size 12-14 red bodied, always worth a chuck out with a PTN on the point.
Yellow Owl CDC – small yellow body with grey CDC “breathers”, size 14-16
Sedge – Brown fly very buoyant and good for browns, size 12-16
Shipman’s Buzzer – I’d personally go either black or orange on this one but it depends what’s catching, sort of a fuzzy dry with white “breathers” front and back
Lakes – Nymphs and wets
Again with the PTN, Hare’s Ear and Diawl Bach but throw in some colourful flies too, Cat’s Whisker, White Bunny Leech, Green and Black Fritz (Green fritz body, Black marabou tail)
Also worth trying out some blobs and boobies and stripping them back as fast as possible, if lots of competitions are held there, sometimes they switch onto the big bright blobs and it’s hard to get them off them again (Not what I’d deem fishing but if needs must)
Sedge’s, CDC’s, Emerger’s, Parachute dries if just lipping below the surface, or try a size 16 buzzer and put some Gink on the line about an inch above it, it’ll suspend the buzzer just through the surface film and entice the fish to take it.