Black gnat 16.
The black gnat's in Scotland are fatboy 16's; in the West Country super-skinny 18's. Go figure. I've only actually tied the parachute version for the box - 19 out of 20 trout say that's what they prefer. Occassionally one wants a "conventional" hackle.
I used to fish the Water of Leith in Edinburgh during black gnat time, with this fly you can give 80 fish days. You need at least half a dozen flies per day, maybe more.
Tying instructions:
black thread, black seal, white poly wing, black parachute hackle.