Positive:
* Highly: (Highly legitimate - very credible and valid)
* Truly: (Truly legitimate - completely genuine and authentic)
* Absolutely: (Absolutely legitimate - without any doubt or question)
* Perfectly: (Perfectly legitimate - completely acceptable and valid)
Negative:
* Illegitimate: (Not legal, valid, or authorized)
* Pseudo: (Falsely claiming legitimacy, fake)
* Quasi: (Having some resemblance to being legitimate, but not fully)
* Un: (Not legitimate, invalid)
Other:
* Potentially: (Possibly legitimate, but not yet confirmed)
* Presumed: (Generally considered legitimate, but not proven)
The best prefix will depend on the specific context and meaning you want to convey.