I have used CSL network for many years (Sunday -> PCCW -> CSL). Switched to 3HK 4G 21Mbps a year ago (due to CSL was asking pretty high rate upon contract renewal). While CSL network is quite good in general, 3HK (improved a lot during the past years) is also a good to go!
Regarding SMT, recently got the birdie free sim ... the reception quality is only "一般"
Network quality / stability :
CSL > 3HK > SMT
No idea about CMHK, but guys saying it's no good ... or even bad ??