I assume you've contacted Optus and cancelled it as detailed here?
What's best probably depends on what your plan details are and how far into your contract you are.
1. Was the phone on a lease plan or standard plan?
2. We're you paying off the phone? What model was it?
3. How long left on your contract? How long was your contract in total?
4. Do you want another similar phone or would a cheap one be ok for the duration?