If we're recommending pet favorites, the Raspberry Pi Zero Wireless, despite being the newest, is now my recommended "microcontroller". There are a few tweaks needed on top of the base Rasbian distro to make it appropriate for use as a microcontroller, but with a base price of $10, it's cheaper, more powerful, and has accessories (ie - anything USB with open source Linux drivers), than some Arduinos.
Built in wifi, Linux, plus USB and GPIO, means it's perfect for the device end of an hobbiest-level IoT project.
I still consider that a 'computer' and not a microcontroller.
You will inevitably need adc functionality, possibly real time clock and battery usage.
Most servos don't need wifi or Ethernet. Instead, you will usually want to be able to read a voltage and then time how long it takes for that to change.
15 cent quartz crystal and built in adcs. Bitbanging pwm from raspberry pi is also horribly inaccurate... You need a ton of shields to just run a motor with an encoder on it... While arduino can do all that around 10mA
The "production" competitor to the Raspberry Pi Zero is honestly something like... the Octavo Systems OSD3358. At $50, you may wonder why this device is superior... but it has integrated RAM, Open Documentation, the features that are necessary for microcontroller work (ADC, DAC, PRU/Programmable Realtime Units, RTC, on-board LDO Regulator, on-board Lithium-Ion cell balancer and Lithium-Ion manager, support for TWO Crystal Oscillators, 114 GPIO, 6 UARTs).
Its all about your REQUIREMENTs. The Raspberry Pi Zero barely does anything aside from computer things (Ex: WiFi, USB).
Don't get me wrong, the Raspberry Pi Zero is a wonderful computer. But microcontrollers are things that can work with Thermocouples, control Active Filters, and control motors. In contrast, the Raspberry Pi burns itself out if you put an LED on its weak GPIO pins... and even "Realtime" Linux can't save you from the micro-second delay associated with the GPIO pin hardware.
My point is that if I needed to build and sell 200,000 of something over 10 years, I can't order that many from the Raspberry Pi foundation and trust the piece will still be around the entire time. And if I wanted 200,000 of the Broadcom parts, they wouldn't sell it to me in that small of a quantity.
The OSD3358 is definitely the right approach, since putting high-speed impedance/timing matched DDR lines on a board to make it work right with a modern 1GHz+ CPU is a much higher level of difficulty. Plus you will also need to make your PCB 8 or 10 layers to shield the interference which increases the cost.
I thought package-on-package (like the AM335x's predecessor, the OMAP) was the way to go, but I'm starting to like the monolithic package better.
I'm always a little cautious with RasPi gpio - it's so much easier to accidentally fry a RasPi than an Arduino, most thing I make that need gpio at least start out in prototype stage using an Arduino to deal with io with a RasPi (or NextThing CHIP) hanging off the side if I need more computrons than an Arduino provides.
Built in wifi, Linux, plus USB and GPIO, means it's perfect for the device end of an hobbiest-level IoT project.