Hmm,
I think I have to split this one up, lets stick to trout.
I had a day on Elk Creek where I landed 15 steelhead, and lost 10-15 more. By weight, this was certainly my best day ever for trout. I know locals do better on occasion, but I don't get up there that often.
I've had a couple days in central PA with 20+ wild trout, one with 30+, averaging 10-12 inches with a few 15-16" thrown in for good measure, and all on dry flies. Tough to beat that.
An evening on Spring "coaching" my wife right after a big storm and a during a sulphur hatch. Perfect conditions, water stained but not unfishable, and noone around because a half mile downstream near the road it was unfishable. I think she got 6, but must have missed 30 more (I'm not a very good coach
). I caught one, on one cast that was merely intended to be a demonstration.
Several with 20+ native brookies in some beautiful surroundings.
My first fly-fishing experience. Oil Creek as a teenager. Carried my dad's typically unused fly rod and a spinning rod. Hatch came off, figured I'd give it a try. 40-50 missed trout. Only hooked one, my first on a fly, but I was hooked too.
Its hard to choose. Other things are perhaps more important to having a good day than the final tally. The company of family and friends is most important. The surroundings and scenery are just as important (I've fished Idaho and Wyoming, and still love a brookie jaunts and big water in nice areas like Penns). And the satisfaction of having "figured out" a complicated situation on a stream like Penns holds a special memory, even if your final tally doesn't rank up there with the best ever.