ns2 is in my opinion the best suggestion, but may be harder to use. If you have a long-time project, it is worth investing the time to learn it, but if you just want to test a short-term idea the overhead may be too much.
Any large simulation tool takes some time to learn, so ask around to local colleagues or supervisor as is best if you know someone that is familiar with it and can help with configuration files etc. Obviously, there is always online help. It is also a good idea to look at the literature and the publications you are aiming for and see what tool is more popular there. This can be part of your literature review.