I agree with what the others have said, no one rod weight is necessarily best, because of the variables. Smallmouths are my favorite freshwater fish, and I've used from a 3 wt to an 8 wt to chase them. The larger the waters, the heavier I'll go, and of course on smaller streams, I'll go light. The rod sizing will limit fly sizes, however most waters will also, and the same for rod sizing will hold true for flies. The bigger the waters, the bigger the flies can be. That does not mean small flies won't work on big waters, or that you always need small flies on small waters. That's where a good selection of flies that cover the range of sizes, that can be cast with any weight rod you use comes in handy. (I've got about a bazillion flies!
For most of the SM fishing I do, I'll use a 6 wt. I started fly fishing many years ago with a fiberglass 8 wt, which I used for several years & that's what I caught my first SM on. Depending on conditions, which is primarily wind & current, I would still use an 8 wt on larger waters such as the Susquehanna or the lower sections of the Potomac that hold SM's. Those are relatively wide open rivers, and hold enough fish of a size that an 8 wt & larger flies can be very productive, and as has been said, an 8 wt may be a bit over matched for smaller fish, but that will be true for most any species. I also would not hesitate to use my 6 wt on those same waters, but I know that would limit my fly selection somewhat. There is also the possibility of other species that would be over matched for a 6 wt. I have at times used my 6 wt with much larger flies than I would normally, such as a 4" streamers tied on 2/0 hooks. It can be done with a very short, stout leader/tippet, but does very much limit casting distance. I once encountered a school of large Striped bass while fishing for other species, and all I had was a 6 wt. The Stripers were in close to me, so distance in that situation was not a big issue, but fly size was. Compromise & adaptation is a part of fishing.
Not part of your question but fits with what I just said about the Potomac. I use from a 6 to a 10 wt regularly for LM bass. I'm most often fishing for them in tidal rivers & creeks, where an average fish may be 2 lbs, and large fish are 5 lbs and over. The heavy weight rods are not needed for the size of the fish, but, there are also other species that I might encounter, such as Striped Bass, or possibly Carp, that can be large enough to warrant the heavier tackle. Plus, I'm fishing in some really snag infested areas, which consists of Spatterdock pads, overhanging & downed timber and various vegetation, which creates problems for getting even a 2 lb bass out of that mess with a lighter weight rod. I also like to use larger size flies in these areas, flies that may be up to 8-10" long tied on 2/0 to 3/0 size hooks, for which the heavier rod is needed. I'm often using 20 lb test tippets, again not due to the fish size, but due to conditions. It's akin to the bass angler who uses 65lb braided line on a baitcaster for flipping jigs, which I also do, because that strength line cuts thru vegetation & aids in dragging hooked fish out of a mass of debris. It has little to do with the fish size in this case.
If you only have one rod, or are only purchasing one rod for general use, then there will always be trade offs no matter what species you chase or where you intend to be fishing.