Without further specifying what you will send (size, frequency, latency requirements) you will get a wide range of answers.
In FPGA you can pretty much write an implementation for SPI, UART, parallel, etc. given that you're proficient enough with the protocol. You may want to choose to have the microcontroller firmware the easiest, i.e. make that a SPI master, or use an UART.
But really it requires much more detail to give any non-generalized advice.