There are mainly four mobile service provider in Malaysia:
Celcom, DiGi, Maxis, U Mobile
You may check the coverage on the map for the location that you want to visit in Malaysia. (Service provided by OpenSignal)
Most of the rural areas Celcom is the best. For example Celcom has the best coverage for Langkawi. For Kuala Lumpur, almost all of them are same and usable.