The best shooter system, IMO, is probably Saturn. Not just because of RS, but also DoDonPachi, the Gradius, Salamander, and Parodius packs, Thundeforce V, Batsugun, etc.
The DC shooters are nice, but most of them are all from the same era, and have the same bullet hell style. The PCE has a lot of shooters, but they are also from the same era for the most part. The Saturn has the best range of shooter styles, IMO.
Certainly any shooter fan will have all three systems though.