Minecraft wiki entry for Building_a_metropolis, associated text: Large downtown street:
Recommended material: Stone, obsidian, or other road-like materials.
Terraform any and all hills, this should absolutely be no more than 1/6th slope with very few bumps.
Don't put these in on a mountainous city unless they are being supported on columns, they look silly when they are super steep. These should also be fairly straight, and never next to houses. Put large buildings near these.