General antonyms:
* Principled: Someone who acts based on strong ethical beliefs, rather than taking advantage of situations.
* Honest: Someone who is truthful and fair in their dealings.
* Scrupulous: Someone who is extremely careful and thorough in their actions, especially regarding ethical matters.
* Unselfish: Someone who puts the needs of others before their own.
Antonyms related to specific contexts:
* Idealist: Someone who focuses on lofty goals and values, rather than pragmatic advantages.
* Altruist: Someone who is selfless and puts the well-being of others above their own.
* Selfless: Someone who does not act out of self-interest.
* Loyal: Someone who is faithful and committed to a cause, person, or group, even when it's not advantageous.
The best antonym for "opportunist" will depend on the specific context and the meaning you want to convey.