The ATTiny85 is a compact 8-pin Microcontroller from the AVR 8-bit family. It is a low-cost controller that is commonly used in basic electronic designs where little programming is required. The controller operates on 20Mhz frequency with a memory of 8K. It also has  4 10-bit ADC for interfacing analog sensors or measuring analog voltage. 

The ATTiny85 microcontroller is popular because it can be easily programmed using the Arduino IDE. If you are looking for the cheapest microcontroller that supports Arduino programming then ATTiny85 might be your choice. 

Specifications of ATTiny85:

• 8-bit RISC Architecture
• Operating Voltage: 2.5V to 5.5V
• Flash Memory: 8kB
• Operating Frequency: 20MHz (max)
• I/O Pins: 6
• External Interrupts: 6
• ADC: 4-channel 10-bit ADC
• Timer: One 8-bit Timer
• EEPROM: 512kB