With advancements in saved/pretrained models I think answer is or will be a raspberry pi. I think a more interesting question is “what is the smallest computer that can learn chess”
I can't find any recent experiment, but here https://en.chessbase.com/post/komodo-8-the-smartphone-vs-des... is a test done in 2014 that shows that smartphones were already clearly superhuman then
In 2019 I estimate we would need less than half of the computing power of that 2014 smartphone to achieve the same playing strength