We sure love the ATmega328 here at Adafruit, and we use them a lot for our own projects. The processor has plenty of GPIO, Analog inputs, hardware UART SPI and I2C, timers and PWM galore - just enough for most simple projects. When we need to go small, we use a Metro Mini, but when size isn't as much of a concern, and a USB-serial converter is required, we reach for an Adafruit METRO.