better/cheaper ways than WiFi to communicate wirelessly, e.g., IEEE 802.15.4. WiFi's 20MHz channel is a big disadvantage for low rate data.RonBeyer:
Its difficult to recommend a MCU without knowing what you need to do with it. I’m assuming it needs to do more than talk on WiFi, what other devices are you talking to? Requirements for A-D or D-A? If you need the most basic of MCU’s, I would go with an ATTINY which come about as small as you can get. Also, how much code do you need to run on them? Do you want to run the TCP/IP stack on the MCU or have some kind of stand-alone wifi device you talk with serial?
XBee Series 1 is a good way to go, instead of WiFi.
Get a pair.
There are cheaper yet, but not plug-and-play.