What is my mother tongue?

Coming out to work as a clerk in the operations side of a big Asian bank, I find the Chinese is the most commonly spoken language amongst the Chinese staff. Which is a language I can communicate rather well in, but its not my most proficient language. As 2 years of army proved.

So I'm just wondering, what is my mother tongue? Yes, Chinese is the language of my ethnic group but its neither the language I first learned at home nor is it the language I'm most proficient in. My dad's Chinese is atrocious, and my mum's is decent. When my sisters and I were younger, she tried making an effort to speak to us more in Chinese given that my Dad clearly couldn't, but as we grew older, it simply became easier for everyone to use English in my home.

As Wikipedia suggests, there's no strict definition for "mother tongue", but highlights Singapore's definition of "mother tongue" as the language of one's ethnicity. I've been brought up with that definition such that I only found out today about all the other definitions of mother tongue. Seems like its the only definition that allows my to state Chinese as my mother tongue. By all other definitions, it would be English. With that Singaporean accent. Whatever it is.