If I was locked to using one fly for the first few weeks of the season, it would be a bead head nymph of some description. To be honest, so long as it carries that 'buggy' appearance and it will fish deep, it will take fish.
On the Wellow, my main haunt, there are plenty of deeper runs and holes where a neatly pitched fly will result in a welcome brownie. Often takes are very subtle, but a slight deviation in the leader must be met with a lift of the rod. IF it's not a fish then the fly can be rolled straight back into the pool. Working methodically with one fly is all that is needed during these cooler times - where there is bound to be plenty of water.
So, here is a selection of bead-head flies added to the box. They are all based around a similar theme, with slight variations:
Roll on the 1st of April..