General:
* Primarily
* Principally
* Mostly
* Chiefly
* Largely
* Predominantly
* For the most part
* In the main
* By and large
More specific:
* If referring to a majority:
* Overwhelmingly
* Significantly
* Substantially
* If referring to a specific aspect:
* Specifically
* Particularly
* In particular
* Above all
The best alternative will depend on the specific context and the nuance you want to convey.