Pan-fish will take most of the imitative trout flies, I've had good success with Pheasant Tail Nymph (PTN) especially with a yellow body but they quite happily take off the top and a Blue-Winged Olive (BWO) or Elk's Hair Caddis are good flies.
Small or largemouth bass? If you are fishing for pan-fish then you're probably in a stillwater aiming for largemouth. I've had good success popping a muddler minnow across the surface but you have find the right structures holding the bass. You can get specialist bass bugs, which are big and ugly and need quite a heavy rod to throw, including Frog/Mice replicas, but I have no more success with these than a muddler. Alternatively your clousers, wooly bu66er or I have found some grubs (waggle tails) light enough to throw with a fly rod (no weight) - though the purist in me struggles to reach for these plastic baits - but when needs must

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Regards
-A.