What is the best coolant to use?

The past 3 mornings when i have came out and started the car the coolant level to low sign is showing on the dash.
It is only showing first thing in the mornings which is confusing me a little, should i be concerned over this? I'm thinking it only shows in the mornings because it when the coolant is at its coldest, therfore at its lowest level.

Thanks Guys