Here are some points you might ponder:
Bullseye is an old "standard", as is Unique. Bullseye is much faster burning than Unique, and better suited to lighter loads than Unique. If you want to load in the magnum, or near magnum class, Unique will do much better than Bullseye. Conversly, if you want to load very light target loads, Bullseye is much better suited. Both are dirty. Unique, is not nearly as dirty if loaded to the higher pressure end of it's spectrum. Not necessarily a max load, but at least medium hot. I haven't used Bullseye in years, so I'm not sure if it cleans up any at higher velocities or not.
If I were ordering and I were going to use just one powder, it would probably be Clays. (There, someone did finally mention it) It is great for mild to medium hot loads, extreamly clean, meters well, etc. Also pretty cheap. If I wanted to load on the hotter end of the spectrum, Unique might get the nod. There are some of the newer powders that might do a better job, but may not be as readily available and don't necessarily have the tons of load data avaialble that Unique has. 231/HP38 is also a viable option for a one-powder loading bench. Just tends to be quite dirty at low pressures. Any of the mentioned powders would be a proper choice, though the only way to find the best choice for you is to try some different ones and see. You will not go wrong with any of them.