Here is the list of some IoT development boards that are perfect for prototyping. There might be some better boards, but these are ones that I personally used.
1. Raspberry Pi
If you are a techie and haven't heard about the Raspberry Pi, you are probably living under a rock. Raspberry Pi is a credit card sized computer that is really cheap. It can be connected to Monitor using HDMI and it can also use a normal keyboard and mouse.
So, how is this useful for IoT? Connectivity options. It comes with Wireless LAN and Bluetooth Low Energy (BLE). There are various ports to connect sensors and actuators. Another great thing is, it supports various Operating Systems. There are various variants of this, the recently released one is Raspberry Pi 4.
2. Arduino Uno
This is another board everyone starts with. Because it is really really easy. It is also cheap and fun to work with. It comes with its own IDE (Integrated Development Environment).
Some variants of Uno (Arduino Uno Wifi) comes with ESP8266 (WiFi module) attached. This is one of the good choices if you like to code with C++. However, there are workarounds to code in other languages, but C++ is preferred.
ESP chips are manufactured by Espressif Systems. There are tiny compared to others and great for IoT applications. ESP32 is an advanced version of ESP8266.
If your application needs minimal processing power with an internet connection with an inbuilt WiFI module, ESP8266 is a no brainer. For bigger projects, ESP32 is a great choice as it comes with a lot of in-built features.
Another innovative board that is almost as big as ESP8266. It's easily one of my favorites. It comes with its own edge-to-cloud platform. This simplifies everything so you can directly concentrate on building your application.
Particle provides Hardware, Operating System, Connectivity options, and cloud services all in one place. It has some boards like Photon, Argon, Boron, etc. It is very useful for reducing the time to market considerably as the whole IoT stack is available to you.
Choosing a suitable chip for your projects depends on what you are planning to achieve with the project. All of the above are easy on the pocket and made for IoT applications. One can choose between microcontroller-based boards like Arduino for simple applications, Single-Board Computers (SBC) like Raspberry Pi if you are a Linux enthusiast, or even cellular-enabled IoT boards from Particle if you plan to take it a level above. Good luck with your next project!