score:5
To the best of my knowledge this is impossible, or at least impractical. Driving Malaysian cars in Singapore is intentionally made hard (cars are taxed much more heavily in Sg), and rental agencies will generally not even let you cross the border, much less return cars on the wrong side.
Your options are:
Upvote:1
I'm surprised at jpatokai's comments re difficulty of getting from Johor Bahru to Singapore. I'd have no hesitation dropping a car off in JB and catching a bus to Singapore if that was necessary. I'd guess it would take half a day max from car dropoff to being at large in Singapore and possibly rather less.
I've travelled once by train from a station half way down Malaysia (closest to Malacca) and done one day trip Singapore-JB-Singapore and border crossing was not eventful or overly slow in either case. You can catch a bus from JB city centre to Singapore and large numbers of people make the return trip one way or the other daily.