Streaming audio to ESP8266 over WiFi
Which pin of the ESP8266 chip can be used as I²S output?


I uses GPIOs 2, 3, and 15. You only have GPIO2 available, and that's also one half of the UART interface.  Also you usually need to generate a "master" clock (usually something like 256x the bit clock) for oversampling. So you can't with that module.


  Which ESP chip would be the best choice for this purpose?


ESP32, without a shadow of a doubt.


  I know about the MAX98357A which is used to convert digital I²S data to analog. But, for stereo output I need to use 2 of those. Is there any alternative to this chip which supports stereo output by default?


Yes, there are plenty. The search term you want is "CODEC". Personally I use the TLV320DAC3101.

You never use two separate mono CODECs for stereo, since I²S is a (normally) stereo protocol for connecting to a stereo CODEC. Reliably splitting a stereo I²S stream into two mono ones is not easy.


  I have found many tutorials which shows how to play audio from SD card of ESP8266. But, how can I stream audio over standard WiFi connection?


By implementing a suitable protocol, and that depends entirely on what your audio is and how it is generated.