Personally for me, and i use both maxis, celcom and digi. (No internet for Digi, gila ah)

I find in terms of network coverage by location, Celcom is better, in terms of location being covered.

Stability, both ok. although Maxis seems to drop more than Celcom.

Speed wise, Maxis is definitely faster a bit.

Taking aside customer service, which frankly i have bad experience with all 3 of them. (So in general, telco customer service sucks), I think if you're travelling a lot, celcom is a better option. but if you're mostly in kl / pj / selangor state area, any of them should do. Since they all cover klang valley well.