The D1 Mini is my favorite ESP8266 board right now. It has built-in WiFi of course. It is physically smaller than a NodeMCU, but still exposes enough GPIO pins

The D1 Mini is my favorite ESP8266 board right now. It has built-in WiFi of course. It is physically smaller than a NodeMCU, but still exposes enough GPIO pins to allow interesting peripherals. The built-in micro-USB connector and USB-Serial chip means that programming it is easy, it doesn't need a separate programmer like the ESP-01. Unlike the ESP-01, the D1 Mini exposes the GPIO16 pin (aka D0) which is required to wake the board from DEEP SLEEP. The ESP-01 requires a soldering hack to enable DEEP SLEEP. The LDO voltage regulator on the D1 Mini is so good that in DEEP SLEEP mode, it consumes only about 200 microamps, so I use it directly in battery-operated applications. In comparison, the ESP-01 goes down to 40 microamps in DEEP SLEEP, so the overhead of the voltage regulator is only about 160 microamps. (Edit: Both of those numbers include the DHT-22 temperature sensor). My outdoor temperature sensor uses a D1 Mini with 3 x AA NiMH batteries feeding the voltage regulator, and it lasts 4 months. It wakes up for 5 seconds, every 15 minutes, reads the temperature and battery voltage, then transmits the info to an MQTT server. In theory, the ESP-01 (w/ soldering hack) would last 6 months on those batteries, but the difference isn't worth the extra hassle of an ESP-01 for me.