I have always seen 'THE ONE' with a huge gamerscore like 100k or over. And I guess it is either random or first come first serve. Haven't played since BETA mind you.
it explains in the terms and conditions how they pick them
Subject to the participation limits for the role of “The One” is described below. Before each Round, we will select the top player in the game lobby to compete as “The One” based on pre-determined score criteria that alternates every three rounds as follows:
SEASON: Your current accumulated score for the entire season.
WEEKLY: Your current accumulated score for the week.
PREVIOUS ROUND: Your score for the previous round of play.
The One for each round will be the player with the highest score based on that Round’s designated score criteria.
We will determine your score based on your then current gameplay statistics. For purposes of this Challenge, we reset your weekly gameplay statistics to zero every Saturday at 06:00 GMT (10pm PST) UK time. Your Season score would still be in place for the entire season
Selection of “The Mob”
After selection of The One as described above, and subject to the participation limits described below, before each Round, we will select the top one hundred (100) players in the game lobby to compete as a member of ‘The Mob’ based on the same pre-determined score criteria and tie-breaker criteria used to determine The One, as described above. However, to qualify for the The Mob, players must also meet this additional criteria; the must not have answered incorrectly as the Mob in any previous Round and they must not have already won any prize this season as the Mob.
Players may compete as a member of “The Mob” until such time as they answer a question incorrectly or win a prize. In the unlikely event that all players in the game lobby prior to a given Round have already won a prize or answered incorrectly” we may relax the foregoing gameplay limitation and allow them to compete in The Mob for prizes.