Sanur would be my personal preference as it is quiet. However, at your age I think you would have a better chance of meeting more people your own age in Kuta or Legian. Legian is nice and has many good places to eat. I have only stayed at the Puri Raja in Legian which was reasonably priced.

You will love Ubud where there is lots of good accommodation for all budgets. I recently (last year) stayed at the Tegal Sari which was good value.

There is plenty to do and see in Ubud and it is a good base. Hiring a driver will not be a problem.

Have a great holiday