I'd say the Teensy is in the Arduino++ camp there. It uses the same libraries, etc, just has more power processing, a pretty nice price, and some interface bits and pieces that are very nice, like being able to act as a usb device and host. A lot of hardware, I'd argue is better to be treated as a library -- there are libraries and hardware that have definite feature sets, some of which overlap, that allows you to choose the best tool for the job.
The ESP8266/Wemos can run Arduino too. Over the summer I built a wifi clock, but couldn't get any of the native LCD libraries for the ESP to work. I flashed Arduino and used the LiquidCrystal library, initially just to make sure I had wired stuff up correctly. But then it worked, and I decided to stick with it :)
Best feature on the Teensy IMO is usb native, so you can do stuff like emulate a USB keyboard or mouse. Many DIY keyboard projects use them as the brains. https://www.pjrc.com/teensy/projects.html has more ideas. Also works great for larger Ambilight displays than Arduino can handle.
