Electronics, Arduinos and Radios
Arduino Pro Mini
As cheap as it gets! Arduino Pro Mini clones are probably the cheapest solution for semi-permanent solutions. Small and extra cheap. Arduino Pro Mini lacks USB-to-serial converter, but that is not a big deal since you probably do not need USB after flashing and setting it up. And you can do development on other boards with USB. It comes in two versions: 5V 16MHz and 3.3V 8MHz.
Arduino Pro Micro
Arduino Pro Micro is a slightly more advanced version of Pro Mini. It keeps a similar form factor but brings a few changes: ATmega32u4 instead of ATmeage328 and dedicated USB ports. On a hardware level, different MCU offers more pins with hardware interrupts and in general, better experience. It's more expensive than Pro Mini, but it's easier to develop thanks to USB port and in general is a more advanced design.
DOIT ESP32 development board
When Arduino and AVR based development boards are not enough, ESP32 to the rescue! Much more RAM, flash, 2 cores, WiFi, Bluetooth, and a flexible IO system. Can be programmed with C, Python, Lua and even Arduino. They are super cheap for the value and allow to make amazing DIY projects. You can read more about ESP32 on this site as well.
CDEBYTE E32-868T20D LoRa serial wireless module
CDEBYTE E32-868T20D is the replacement for discontinued E45-TTL-100 that I was using for some of my projects. It is equipped with the sam Semtech SX1276 LoRa 868MHz radio chipset and can easily do a few kilometers on the ground level. Relatively simple to use, since it works as a serial passthrough and you do not have to program the RF chipset itself. More about this kind of modules is available here.
TTGO ESP32 with OLED display and LiPo charger
Oh, did I already mentioned that some ESP32 development boards come with build in OLED? It might not be big since it's only 128x32 pixels, but it's already there. It is also possible to power this board using 1S 3.7V LiPo battery? It even as a built-in charger, so the LiPo will be charged when USB power is used!
You think Bluetooth module, you say HC-06. OK, maybe it's an overstatement, but truth to be told, HC-06 Bluetooth modules are the workhorses everywhere where the serial-over-Bluetooth connection is required. They can be programmed with AT commands and be used for various applications.
RCmall LoRa32u4 868/915MHz Arduino LoRa
You want to start with wireless communication and LoRa? Go with LoRa32u4. It uses well-known ATmega32u4 and is 100% compatible with the Arduino platform. Built-in SX1276 compatible RF chipset can give a few kilometers of range. My Crossbow DIY LRS radio system is built with those boards.
LORA32 LoRa ESP32 OLED V2.0 868/915Mhz
ATmega32u4 is not enough? Looking for something more powerful with WiFi, Bluetooth, and LoRa? Why not TTGO LORA32 v2. The same LoRa chipset as in LoRa32u4 and can be used almost like regular Arduino board. Oh, it has an OLED too. SD card slot and can be run from a LiPo. Awesome!
HC-12 433MHz serial passthrough module
Another wireless serial module. This time it's not LoRa but good old FSK. Although it uses "older" technology, it's still a quite good choice when you want to send some data over the radio and do not program RF chipset directly.