Hmm... if you just want to be entertained and aren't looking for classic cinema I'd vote for:

* Needing You

* Love on a Diet

* 无间道

* Let the Bullets Fly

* Balzac and the Little Chinese Seamstress

* Big Shot's Funeral

* 有话好好说 (?)

* 宝贝计划 (Jackie Chan's baby/action comedy)

Not all mainland movies, but they should all have mandarin soundtracks.