For me Espruino seems very flexible. It runs on many Chip-sets, ranging from cheap ST discovery boards to ESP8266 Wifi boards (Adafruit Huzzah).
It has lots of examples and running the examples is a button press away.(The examples have a little button that imports the example code into the Chrome IDE app)
Currently I am playing around with it on this nifty little board : https://www.tindie.com/products/onandoffables/arminarm/
It plugs into a Raspberry Pi and exposes a few option to play with ARM chips. (Espruino, Lua, Maple, CMSIS)
The Espruino board is also very low powered and can run on a few uA's.
Tessel - Does a lot of the same stuff as the Espruino. Has more memory,is faster,has WIFI. Can do other languages like Python and RUST. Is more like a Raspberry Pi. It has modules where Espruino uses standard components for sensors (like Arduino).
Arduino Yun - Its an Arduino + Wifi module. ESP8266 does the same thing for much cheaper. The Yun Wifi Module actually is a small Wifi router with an OS running on it(OpenWRT from what I can recall).