Which is the best memory option to store about 8MB of sensor data?
You can do it in several ways
SD/uSD card:
++ detachable.
++ connected via SPI
~~ medium power consumption
Use it with FatFs module by ChaN. There is an example on his site for stm32.
Dedicated IC, AT25DF641 (8MByte) for example.
-- undetachable.
++ connected via SPI
++ lowest and predicatable current consumption.
You can you it by addressing flash sectors or by installing a filesystem, spiffs for exmample.
USB-stick
++ detachable
-- high power consumption.
-- Unpredictable power consumption
-- Need to get into USB stack.