My favorite is probably the Sulphur spinner fall - theres nothing quite like casting dries to rising fish on a nice warm spring evening while the sun is setting.
However, the Large BWO hatch is a close second - I've caught some of my biggest trout early in the morning on these flies.
Friends and I used to stay at a campground outside of Wharton and spend eight solid days of (drinking) fishing 9am-9pm. Second year I went, we timed the march browns perfectly and absolutely crushed fished all week on big size 10's and 12's. That trip is what caused me to fall in love with fly fishing over ten years ago. So if I could only fish one, it would be march browns. Close second would be green drakes, anyone who's cast over fish who are keying on them knows why.
Any. Sulfurs are hard to beat for predictability. March Browns are great size and numbers. Drakes are epic if you like sport utility bugs. Grannoms are like a confetti drop.
And- I witnessed a Hexagenia Limbata hatch last June. I was unprepared that night, but went back two nights later with much bigger flies and scored. It was amazing seeing a lifeless stream turn into a feeding frenzy on those giant mayflies.
On an annual basis, probably Grannoms. It seems to be the kick off for dry fly fishing for me each year. They're often keyed in on some movement. It's pretty exciting giving the fly a well timed twitch and inducing an eat.
Some of my favorite times fishing have been March Brown hatches. It is not a hatch that I have been able to hit reliably, though.
Personally, I will say Cicadas was probably the best hatch I’ve ever experienced. However I’ve seen some amazing PMD/and or sulphurs in tailwaters that were a very close second.
But as a yearly hatch, I absolutely love prospecting with a March Browns. Something about thinking “that spot looks fishy” and then being rewarded with a take and a great fish… there’s just something special about that. And there’s a particular stream that I have in mind that I enjoy more than others.