I'm going to limit this to just dry flies, because there are so many soft hackles that fill the bill that I can't pick just one.
It's an easy choice with dries, though: a Renegade in a size 16. It's midge cluster, it's an ant, it's a caddis, and it's any number of other things. It even works fished wet, swung, or fished like a nymph and even stripped like a streamer. I'd estimate that 80% of the fish I've taken on dries in the last seven or eight years (and that's a lot of fish) have been on a Renegade, and 100% of those taken on a dry in spring creeks.