Nikon d3100 iso settings. What do you use?

Title says it all rly.

What is a good all round iso to use for day to day.

Oh and which is best for night shots - city scapes ans stars ect.

Always use the lowest practical ISO setting to minimise noise, or speckling, in your pictures. The higher the ISO the more noise.

http://aviewfinderdarkly.com.au/2007/05/25/iso-sensitivity-and-noise-in-digital-photos/

Always (always, always, ALWAYS) run as low as an ISO as possible. In between 100 and 400 is where you want to be. Anything above that, and you are going to see a lot of weird stuff in your pictures. If you are taking a night shot, still keep the ISO low if possible, and take the picture using aperture and shutter speed combinations. Unless, of course, you wanted the noise from the ISO to be part of the aesthetic, and then that's a different story.

PS, 100 ISO on a Nikon is "Lo 0.3," I believe.

The one you need. I'm not even being snarky.

Nikon's "native ISO" is 200 and that is your ultimate desire, however it's useless if you can't get proper exposure at that ISO.
ANY ISO will work, but you need to use the one that you need for the situation you are in.
It is all done by metering and setting your settings according to what you need.
You want to shoot with the lowest possible ISO that will give you PROPER exposure. NEVER underexpose an image in order to use a lower ISO. It will give you worse noise if you under-expose at ISO 200 instead of using a higher ISO.
You have to decide what you NEED for the image and set the exposure triangle accordingly. I know that sounds like no answer at all or really stupid, but that is because there's no one answer. It changes with every lighting situation.

Please don't go by the ALWAYS use the lowest ISO. It's just not true. If you expose properly for the ISO you are using you can use ANY ISO on your camera. It's a matter of exposing properly and learning about what you are doing.