Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
I'd say 80% of the flies I now tie are thorax style with CDC upright wings, maybe 10% are tied somewhat down wing to imitate the emerger, and the rest are some parachutes and a few "Catskill style" just for old times sake.