There are many simulation and emulation tools that can work with IoT such as NS-3, Mininet-IoT, etc. However, i find out that Mininet-IoT is the easiest tool.
NetSim is a popular simulation tool for IoT. I prefer it over the free tools which are either too simplistic or too difficult to use.
NetSim allows for end-to-end simulation - you can connect the wireless sensor nodes through a gateway and then onto routers and servers. It also enables simulations across all 5 layers of the TCP/IP stack.
In addition, users can combine two different technologies such as Software Defined Networking (SDN) and IoT.
Most other simulators provide just the limited functionality of simulating just sensors. And the radio propagation models in other tools are too simplistic while NetSim provides realistic and detailed RF propagation models.
Different types of simulators used to simulate IoT projects. It depends on your project types. I think the following link help you to decide which simulator you can use in your project: https://windowsreport.com/iot-simulators/
It depends on the simulation requirements: protocol level or system level. If it is a protocol level, NS-3 is a good choice. Normally reviewers consider NS-3 as a trustworthy platform. For the system level, I find many excellent answers given already.
The NetSim, and we can find the emulation useful since we can connect it to external devices. And also external interfacing to MATLAB, IBM Watson, Node etc. Hence we are able to set up a comprehensive IoT testbed.