What is the best PABX system

[durian]

Hello treggies,

    In my working place we are in the process of purchasing a brand 
new PABX system.. our major requirements are ACD, IVR, voice mail and 
PVN and CTI

Several brands are under consideration.. can you please tell  me the 
brands in the order of best to fair ?

Awaiting reply, 

TIA

RAMFEL