Formal:
* Particularly
* Notably
* Specifically
* In particular
* Chiefly
* Primarily
* Mainly
* Notably
Informal:
* Really
* Very
* Above all
* Most of all
* In a big way
* More than anything
Other options:
* Notably
* Significantly
* Remarkably
* Notably
* Conspicuously
* Strikingly
The best synonym depends on the specific sentence you're using it in.