I've always had great luck with the plain ole fur ant. Just dub the back end of the fly in a nice ball, tie in the hackle, wind a few turns, tie off and dub another ball up front. Nuttin' to it.
The best size/color combo is what Paulg and his friends call The WMD. A size 18 orange ant with white hackle. Of course when I say orange, I mean orange! Hareline makes a dubbing that is the color of a hunter's vest. That's the stuff. And don't use dubbing wax or it will get dirty and not be as bright. Just moisten your fingers and twist like heck. If you really lay on the pressure when you dub the fur and then when you wind the body segments, it can be a surprisingly durable fly.
Oh, a tip for fishing the WMD. Dress it and your tippet well, so it sits high. And don't trim the bottom of the hackle. I have a suspicion that the trout take it for a cranefly as well as an ant. So if it sits high, it can be quite convincing.