Formal:
* Particularly
* Notably
* Specifically
* Primarily
* Principally
* In particular
* Most importantly
Informal:
* Really
* Very
* Above all
* Most of all
* In a special way
* To a great extent
* More than anything else
Other:
* Chiefly
* Primarily
* Notably
* Significantly
* Notably
* Remarkably
* Significantly
* Strikingly
Examples:
* "I love all fruits, especially apples." (Formal)
* "I'm really tired, especially after that long hike." (Informal)
* "The project was a success, notably due to your hard work." (Formal)
* "This dessert is amazing, most of all I love the chocolate sauce." (Informal)
The best synonym will depend on the context of your sentence and the specific meaning you want to convey.