According to me a good cloud simulator will be one which satisfies all your requirements. so sir you should specify what are the requirements/strict specification needed for practical approach. Otherwise good simulators are Eucalyptus, Opennebula, GreenCloud, iCanCloud, Grid gain
You can use Greencloud, which is a sophisticated packet-level simulator for energy-aware cloud computing data centers with a focus on cloud communications. see: http://greencloud.gforge.uni.lu/
It depends on the cloud requirements that you would like to simulate. Energy: GreenCloud, Network: Mininet, HPC: SimGrid, Service behavior: Emusin, and so on. Each simulator is better in some characteristic.