For describing something exceptional or impressive:
* Exceptional
* Remarkable
* Exceptional
* Noteworthy
* Impressive
* Extraordinary
* Brilliant
* Excellent
* Magnificent
* Superb
* Phenomenal
* Marvellous
* Splendid
* Exemplary
* First-rate
For describing something that is due or unpaid:
* Unpaid
* Overdue
* Unsettled
* Outstanding balance
For describing something that stands out:
* Conspicuous
* Prominent
* Noticeable
* Eminent
* Distinctive
Other synonyms that can be used depending on the context:
* Excellent
* Superior
* First-class
* Top-notch
* First-rate
* Uncommon
* Significant
* Important
* Notable
* Distinguished
The best synonym for "outstanding" will depend on the specific context of your sentence.