Well, I say ESP8266. In fact this one uses the ESP8285, which is basically an ESP8266 with 1MB of flash built in, so no need for an external flash chip. That makes it even cheaper to manufacture, and hence for you and I to buy.
The ESP8266 is everywhere now. Whenever you buy some cheap Chinese (or even expensive non-Chinese) IoT device, like a WiFi power switch, a WiFi connected lightbulb, or whatever, you can pretty much guarantee that inside will be an ESP8266.
Xtensa have pretty much got it made.
But what is it that's so great about the ESP8266 that's got everyone using it?
Yes, I have hacked another ESP8266-based device. This time it's the LOHAS WiFi Smart LED lightbulb.
I recently needed to connect an ESP8266 board up to an audio control and processing board I was working on. And as you can guess the noise was horrendous. Kind of like when you leave your mobile phone near a radio and someone calls you. Pop, pop, crackle, pop pop...
So, some filtering was in order. In the end I came up with this circuit:
The values can probably be refined somewhat, but I just used components I had lying around.