If the scope is on the rifle I use distilled water and an optics cloth (available a any store that sells glasses) or one of the flannel cleaning patches like what you clean the bore with.
If it's off the rifle, I just dunk it under the faucet and use the same type of cloth to clean/dry.
For just the outside of the scope, any soft towel or paper towel and some distilled water. When I put them up for the year I put on scope covers of some sort and use a lightly oiled rag to wipe the metal ONLY.