HO scale probably offer the most equipment available. Probably followed by On30. After that On30? And then G scale like 1:20.3.
If you want something unusual I model 35n2: 1/35 scale, narrow gauge, two foot. In 1/35 scale HO gauge track comes out to two feet. So I use both HO and On30 mechanisms for locos and trucks for rolling stock. It means scratch building just about everything, and handlaying track. There are 1/35 scale military figures that can be modified to civilians. My locos are also BPRC: battery-powered remote control.