Ok, clear @alexgaro5. I think my personal preference would be to connect them via I2C. Mostly because I find it relatively easy to do it this way, but arguable it's not much simpler than a (soft-)USART connection. SPI, as @Idahowalker says - why not? Also a viable option.
Perhaps a distinction can/should be made on the basis of the number of connected sensors and the frequency of updates that you want to send over MQTT, or, in other words, how responsive you want the system to be.