What language do you sugguest?

[glorfindel]

Mandarin is the most widely spoken Chinese dialect. The grammar is very simple, but the spoken uses 4 tones as phonemes (hence, the singsong aspect) which are quite difficult for English speakers to learn without intense practise with native speakers. Most of the time they will be speaking slang anyway, which you won't learn from a textbook.

Having said that, hiring a cute Chinese lady as a tutor might have other advantages.