General:
* Scientist
* Scholar
* Investigator
* Analyst
* Student (if they are conducting research as part of their studies)
* Academic
* Expert
More specific:
* Historian (for historical research)
* Sociologist (for sociological research)
* Psychologist (for psychological research)
* Biologist (for biological research)
* Statistician (for statistical research)
* Engineer (for engineering research)
* Data Scientist (for data-driven research)
Informal:
* Investigator
* Prober
* Explorer
The best word to use will depend on the specific type of research being conducted and the context of the situation.