The Order of preference of services for Civil Service is asked when you have qualified for the mains. This list of services includes the All India Services, Group 'A' and Group 'B' services, you may choose the order of preference of services as per your choice. You might be want to join a particular service which you wish to opt for, but rather than get stuck to the only one service, you should choose other options too, that too in order of popularity. Why to waste your chance to choose the best? You can change your preferences to a less popular service once you qualify, but you will not get an opportunity to change to a more popular service even though your rank may permit it. So it is not a good idea to list the order of preference starting with the service you wish to join? Its better you choose the preference according to the list of services in Demand.
Some people underestimate themselves and list their preference first for lower or less desirable services so that they get it for sure. Why to under estimate yourself when you have qualified for the mains? Start with the very first service that is All India Service then Group A and if you belong to south or the union territories you may choose the Group B Service. Make sure you do not leave out any service due to over confidence. List all of them in descending order so that in case you do not get the service you think you will surely get, you still have some options left. The civil services results can be very unpredictable and even if you are a topper in your school and college, you may not get the best services on the list.
Get to know about the responsibilities and posting of all the services and their standing in society vis-a-vis other jobs you may take up. You may look down upon some services to begin with but most times they are better options than to take up private jobs or other technical jobs. Make sure you do your research and not regret later as many do every year. You can see some basic information on the links above or also take some advice from the Civil Service Exam coaching experts too. You will then be aware about the best of the services which you can pursue and choose while applying for the mains.