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.
It's unfortunate that there are not more narrow gauge modelers. Some of the equipment offered commercially is very finely detailed and runs extremely well. It ain't cheap, but I get a lot of satisfaction from watching it run.
It seems, at least to me, that modeling narrow gauge has been growing over the years regardless of the scale. With narrow gauge you don't need a large amount of equipment to have a functioning railroad. Many narrow gauge lines were built for a specific purpose such as hauling ore or logs. And you have the quaintness and quirky of narrow gauge locos and rolling stock.
If you get away from the D&RGW/RGS or SR&RL one can build an interesting narrow gauge line with a handful of switches and rolling stock.
I model a fictitious two foot gauge line located in the southwest. Scenery is pretty basic needing mostly just sand and rocks. Structures are also minimal.