Of course Mandarin.
It spoken in China, Taiwan and other Chinese people in different countries like Singapore,Malaysia,Indonesia...etc.
About Hong Kong and Macau,their main language is Cantonese but now Mandarin is more important than Cantonese,many of them are learning Mandarin after 1997.
Mandarin is the main official language in Taiwan and China.
There are some other dielects in Chinese.
Will you learn Mandarin in China or Taiwan? Almost when all foreigners are learning Mandarin, go China or Taiwan are the main choices.
China is using simplify Chinese characters but traditional Chinese characters are using in Taiwan,some characters are the same writing some don't.
for example "country"
國家 (traditional characters), 国家 (simplify characters)//////
"conference" ,會議(traditional characters), 会议 (simplify characters)///////
"internet" , 網路<Wang3 Ru4> (traditional characters), 网络<Wang3 Ruo4> (simplify characters) ........etc