Formal:
* Devoted
* Committed
* Assiduous
* Conscientious
* Diligent
* Zealous
* Single-minded
Informal:
* Passionate
* Enthusiastic
* Hard-working
* Loyal
* Giving
Other options:
* Faithful
* Sincere
* Wholehearted
* Unwavering
* Unfailing
The best synonym for "dedicated" will depend on the specific context in which you are using it. For example, if you are talking about someone who is dedicated to their work, you might use the word "committed" or "hard-working." If you are talking about someone who is dedicated to their family, you might use the word "devoted" or "loyal."