Tough one. But I would say GGGB.
In the Zone is by far the better album, that's not up for debate. Britney was also very involved in the making of the album, unlike Rih on GGGB, and people started seeing a potential they didn't know was there. Sadly knee-gate happened and her best era had to come to an early end, and then her personal life got in the way of her career.
GGGB skyrocketed Rihanna to the top, had a long string of hit singles, Unbrella is iconic (a Britney reject, ironically) and basically inaugurated the most important pop career of this generation. That's about all it h