I left the KL Hilton last week, went to the old train station that is nearby, and bought one way tickets to SIN on Nice busways for $R80 a person. Takes about 5 hours. The border crossings were very easy. 3 minutes on the Malaysia side, and 8 minutes on the SIN side.

It did drop us at the Copthorne hotel, which was a bit out of the way for us, but taxis are pretty cheap.

The bus service was nice, with drinks (water and tea/fruit box), and a light snack handed out. Some movies playing, and a 20 minute break about halfway through the journey.

I'd guess there are 6-10 different companies providing the same service.