“all of the above”
Many people will answer “all of the above”, and most “both sales and marketing”. If this is this case for you then it is best to start with the sales process and then bring the rest on line afterwards. Why? Because marketing and support teams are well disciplined people, used to and happy to accept automation, and they know that their tasks cannot be achieved without a system. Sales people, on the other hand, are perfectly capable of making a sale without a CRM system so their cooperation cannot be taken for granted. So design your system so that it meets the needs of the sales people, and then fit marketing and support/service around that.
For sales people to use the system fully, it must be both useful to them and easy to use, so don’t make the design too complicated. The more complicated the design is, the more fields you add to each screen, the more screens you have to go through to add a contact, the more barriers to successful adoption you will have erected. Every extra field you ask the sales person to complete, especially mandatory ones, the greater the chance that the sales people will enter garbage, leave fields un-entered, or simple only use the system under duress.