This depends how you are using the expression. If you are talking about something you like/see. For example, a man in a Ferrari - you can say "Now that's the life!" - meaning that is the life (how you want it to be). Or if you are relaxing on a beach and enjoying yourself - you can say "Now, this is life!" or "This is life!" - meaning you are content (happy) and expressing that you are enjoying the moment.