My point of view, OMNeT++ is good one if you are looking for an open source simulator. It supports for mobile and wireless networks. Integrate the MiXiM framework for mobility option. QualNet and OPNET are commercial and it can supports for wide range of networks. GloMoSim highly recommended for MANET.
NetSim is more easy and useful for routing optimization purpose. its good feature is that you can interface NetSim with Matlab, sumo, wireshark etc for analysis purpose.
We have been using NetSim for more than 6 years now and have found it well suited for network R & D in mobile and wireless communication. Our students are constantly exploring the features & functionality of the software.
For a networking projects, simulation is often a natural choice. I believe NetSim is the best tool available because its user-friendly interface and rich features.