Great question. I use belts from Bianchi, Galco, and Allessi (both 1.5" and the more dressy 1.25") as well as leather from 5.11 and Bladetech. All serve to support the weight of a holstered EDC handgun and spare mag.
Ensure the belt is nylon stitched and where possible either a domestic manufacturer or domestic manufacturer using Berry standards. This ensures higher quality stitching and materials.
The belt should be wide as possible (1.25 for dress slacks and 1.5" for jeans and cargo pants) but also thicker front to back. Next, grab a sample and try to crush it laterally (top to bottom) to determine its overall lateral rigidity. This is where cheap imported belts fail and means the holster will bend the belt over and eventually flop around. Many modern gun belts add a cord of nylon or plastic along the top and bottom edge of the belt just to add more lateral ridgiity. Older styles use four line stitching or lazy 8 style stitching to increase ridgiity.
Generally 1.75" belts work well but be careful as most pants and holsters aren't made for that width. Also unless wearing tactical cargo pants they have the look of a Santa belt or a police-ish look you may not want to advertise.
Glad to help, DC Reed