The Internet of Things (IoT) represents a new stage in the digital revolution, fully contributing to the construction of a digital society. Several sectors of the digital society use or will use the internet of objects to improve deployment, exploitation and industrialization procedures. These include intelligent transport, smart grids, smart city, industry 4.0, and so on. The issues concerning the mastery of the technologies and processes around the internet of objects are enormous and require a very high level of expertise with cutting-edge skills in multi disciplinary areas.  (http://www.eurecom.fr/en/teaching/master-science/master-degree-internet-of-things-IoT)

I would definitely recomend NetSim (https://www.tetcos.com). Some information on the IOT library is available at https://tetcos.com/iot-wsn.html and at https://tetcos.freshdesk.com/support/solutions/folders/14000105117

The Internet of Things (IoT) represents a new stage in the digital revolution, fully contributing to the construction of a digital society. Several sectors of the digital society use or will use the internet of objects to improve deployment, exploitation and industrialization procedures. These include intelligent transport, smart grids, smart city, industry 4.0, and so on. The issues concerning the mastery of the technologies and processes around the internet of objects are enormous and require a very high level of expertise with cutting-edge skills in multi disciplinary areas.  (http://www.eurecom.fr/en/teaching/master-science/master-degree-internet-of-things-IoT)

Contiki-OS and Cooja is a good place to start for the following reasons:

There are other advantages to using the duo, see

https://books.google.com.sa/books/about/Practical_Contiki_NG.html?id=rv1fDwAAQBAJ&printsec=frontcover&source=kp_read_button&redir_esc=y#v=onepage&q&f=false

I would suggest you go in for NetSim for IOT, since it is the only software which covers the entire IOT stack. It covers

App layer: COAP, Sensor APP

Transport layer: UDP, TCP

Network layer: IPv6, IPv4, AODV, RPL

MAC Layer: 802.15.4

Phy Layer: 802.15.4 with various radio propagation models covering pathloss, fading and shadowing

And also has a well designed and easy to use GUI and a output results windows with necessary output tables & graphs.

Source C codes are available with NetSim for user modification.

There is also a large community of researchers who use it and also good technical support from the developers.

Very good answers!! I have used already Contiki OS. Have anyone used Siemens Mindsphere Operating System?

cooja simulator my students P.G and research scholars using cooja simulator for IOT .

I recommend ifogim for simulating IoT and Fog environments.

Please find it in https://github.com/Cloudslab/iFogSim

In my experience Cooja/Contiki is something to stay away from. It's a almost impossible to get to work (except for their examples) and your are own your own when it comes to support. 

You can start with:

Cooja Contiki: http://www.contiki-os.org/start.html

NetSim: https://www.tetcos.com/

OMNeT++: http://omnetpp.org/

The simplest and most limited - Cisco Packet Tracer 7: https://www.netacad.com/maint.html

I would recommend NetSim (https://www.tetcos.com).

We faced too many issues with these open source tools and got NetSim which is available at a very low price for universities.

Thingspeak and exively,

Or you can build simple testbed using arduino and visual studio as discussed in the following paper

https://www.researchgate.net/publication/309558361_A_WoT_Testbed_for_Research_and_Course_Projects