i think Rihanna has come from nowhere, she had two albums 'Music Of The Sun' and 'A Girl Like Me' which produced singles like 'Pon De Replay', 'SOS' and 'Unfaithful' between them, but it was the biggest selling song of the 00's 'Umbrella' which really caught everyone's attention, and since then i feel she has been the most consistent and successful artist.
However, i feel another artist through the 00's had earned themselves the title of best pop star before Rihanna was even noticed, and that is Beyonce. Although her latest album hasn't been an incredible success, she has survived the 00's better than Britney, Madonna, Kylie, Christina Aguilera and J.Lo and has been very consistent and cemented her place as a superstar.