Probably the black poly pipe that comes in a roll. I would run 1 inch. Some new softeners have 1 inch coonections. You could then reduce to 3/4 or half after that. The new stuff might have a wire in it for Bluestake Purposes/ Utility location in the future. But you might have to tape a wire to it. This pipe uses ribbed fittings that are a bear to push in the pipe. Some silicone grease will be helpfull here and then you tighten hose clamps on it to hold it. You might want to backfill with sand around it at first then the dirt. Rocks will rub holes in it from earth movement. You might be able to use a pex type pipe but unsure.