The "closest" would be the Toccoa, but not a great chance of catching one. I'd bet some people can hammer them on the Toccoa, but most cannot.
The "best" option (in my opinion) given distance/likelihood of catching SM would be the Little Tennessee in Franklin as mentioned above. From beginning of spring, all throughout the summer, you can throw a worm or small crankbait and catch them. They're *generally* wither on the banks or in the shoals. Once you figure out which place they are you can catch plenty. Also, it gets blown out easy but seems to clear up quickly too.
Also mentioned, they are in the Savannah River shoals in Augusta. From Woodstock its 10-15 minutes further than Franklin is. Difference is you'd need a NC fishing license, which is an added cost.
Nowhere on earth that I know of holds a "good population" of predominately 20" + smallmouth. I have caught them up to three pounds or more out of the river. They get bigger on down the river in Fontana Lake. There are some big ones in there, but like any river smallmouth fishery, you will catch a lot of smaller ones for every hawg you catch. Even a foot-long smallmouth will put up a good fight on a light action spinning rig or fly rod.
SAvannah river in Augusta has some. I've never caught them but I know people that have. Best closest fishery I have fished is the upper Broad in SC. I've caught a ton there and seen some good ones landed too.