Pinyin:
* Mòníkā: This is the most common and accurate pronunciation using Pinyin.
* Mòníkǎ: This pronunciation emphasizes the "ka" sound at the end, more like how it's pronounced in English.
Wade-Giles:
* Mo-ni-ka: This is the older romanization system, but still sometimes used.
Cantonese:
* Maau1-nei4-gaa3: This is the Cantonese pronunciation.
Ultimately, the best way to say "Monica" in Chinese is the one that you are most comfortable with and that you think sounds the most natural.