[0xff00ffee]

I'm a big fan of the STMicro nucleo L476RG. Multiple UARTS and SPIs, really low power, custom clocking and deep-sleep modes, 32x the flash of an Uno, and up to 80MHz. Comes in $13 arduino-like board, or a smaller $8 version like an Arduino micro.

[numlock86]

The "BluePill" (STM32F103) is also good, really small and pretty cheap if you don't need the Arduino compatibility. You will need a dedicated programming tool (ST-Link), though. At least that's what I use for more than half of my WS2812B related projects.