It depends a bit on which features you want. The first one I bought was a Lilygo T-Beam. It is an easy device to get started with as it just needs a battery and case, and has WiFi and GPS built in.

https://www.lilygo.cc/products/t-beam-v1-1-esp32-lora-module

I also really like the RAK Wireless devices. They have a modular system so you can customise which features you want and they are very power efficient but they need a bit more work to assemble them.

https://store.rakwireless.com/products/wisblock-meshtastic-starter-kit