Limited info: I've only ever used Celcom, because there is a full-service desk just to the right of where you exit customs. There, you can produce a passport, get your account set up, and purchase any credits. Unfortunately, I can't compare its costs or services with its competitors, but it's served my needs for several years now.

If you're only transiting at KLIA, you might instead be connecting airside and won't exit customs at KLIA. I believe there is also a Celcom booth airside, but I have never used it, so I can't be sure.