What is the most popular embedded serial bus?
If you're uncertain, and your requirements are pretty vague, I'd pick I²C.

The major difference between SPI and I²C is that SPI requires a chip-select line for every peripheral. I²C broadcasts a peripheral address at the start of communication, so it doesn't need chip-select lines. Chip-select lines get cumbersome after the first few.

On the other hand, SPI is probably easier to implement and debug. It could be the winner if you just want to connect to a couple of devices.

I'd rule out USB unless you need high data rates over relatively long distances (m rather than cm). I'd also rule out RS-232 unless it's still 1976 and your peripherals need a massive signal to distinguish a bit from noise.

You might consider Dallas 1-wire, but I suspect it's not as common as I²C, and a "1-wire" bus that needs 2 wires to operate has always seemed a little fishy to me.