From my experiences of taking OC almost every trip, mostly
307.
Let me rank them according to the number of times it is being
played: 
1. "Doors, are closing, ti ti ti ti ti ti" (The
most)
2. "Thank you for travelling with SMRT Buses."
3. "Please hold on to the grabpoles or handgrips when standing
in the bus."
4. "Welcome abroad SMRT Buses"
5. "Please move to the rear for the benefit of boarding
passengers."
6. "Please do not leave your belongings behind."
7. "Attention please, this bus is affected by service diversion
ahead, please check with our service leader for details."
(The least)
lol. Why don't have
"eating and drinking is not allowed in buses" x2
and
"if you see any suspicious articles or person in the bus, please
inform the bus captain(our staff) or call 999.(or press the
emergency button located beside the train doors)
