You can pretty much catch a smallmouth on anything that looks like a meal. Kind of like me, they aren't that particular about what they eat as long as it looks good. I've searched a long time to find the best smallmouth flies. And I've tried a lot of different things. Some were successful for me and some were not. I'm sure you will get a ton of answers but I think you have to find what works the best for you and your style of fishing.
After all of this time and all the different flies I have tried I have two that will go with me everywhere. One for the top and one for the bottom. I haven't settled on a streamer though. Mainly because if that's what it takes then I'll probably be throwing a spinnerbait or a fluke.
For me the best option for up top is a gurgler. The biggest smallies I have caught on a fly rod have come on a gurgler. I think the reason it is the best for me is that you can fish it so many different ways. Like a popper. Just dead drifting it. (not my favorite way either but these are where the biggest have come from). Or skating it across the surface. That is my favorite way because the takes are so explosive. Color combinations are endless.
Gurgler
Now if you don't find any love on the top you can always (usually) find some on the bottom. I've tried hellgrammite patterns, crawdad patterns, worm patterns, blah, blah, blah..... Some have worked at times, some have never worked and some are just a pain in the butt to tie. One thing I'm looking for is something that is quick and simple that's not going to cause a breakdown when I hang the thing 30 ft. up in a tree. So I came up with this. I don't really have a name for it, not sure what it really is. Is it a jig....or a fly......not sure but it does work. We'll just call it the flysmallie pig.
I like the olive but different color combos work as well.
Black/Blue Flysmallie Pig
There are a lot of good patterns out there but it's up to you to figure out what you like to fish with. You are going to catch more with something you have confidence in over something that somebody else told you about.