Based on the level of respect or formality, you can say "My mom is my best friend" in Korean in 3 ways:

• Informal: eomma-neun na-eui jeil chinhan chingu-ya (엄마는 나의 제일 친한 친구야)
• Formal: eomeoni-neun jeo-eui jeil chinhan chingu-ya (어머니는 저의 제일 친한 친구예요), eomeoni-ga jeo-eui jeil chinhan chingu-ipnida (어머니가 저의 제일 친한 친구입니다)
• Very Formal: eomeonim-eun jeo-eui jeil chinhan chingu-isipnida (어머님은 저의 제일 친한 친구입니다)

- my: na-eui (나의), nae (내), jeo-eui (저의), jae (저)
- mom: eomma (엄마), eomeoni (어머니), eomeonim (어머님)
- best: choisang-eui (최상의), jeil natda (제일낫다)
- friend: chingu (친구)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.