There is no singular most dominant. Serena and Maria put in two of the best grand slam performances, with Justine's Roland Garros behind them. Venus' 2000 and 2001 Wimbledon titles are nothing short of impressive as well:
In those two title runs she beat in the top 10: Hingis, Davenport x2, Serena, Henin, and Tauziat losing only one set in to Hingis and one set to Davenport. She beat future top 10 players Sugiyama, Hantuchova, Petrova in straights (giving Petrova a bagel). And she looked positively regal while doing so.
It's not about sets and games won. It's about how invincible you look, and even after losing a set to Hingis and Davenport, I remember not having a doubt that Venus would win.
Numbers on Justine's 2006-2007 RG run:
Sets lost: 0
Opponents in top 10 defeated: Kim Clijsters, Anastasia Myksina (2006); Serena Williams, Ana Ivanovic, Jelena Jankovic (2007)
Other notable opponents: Anna-Lena Groenefeld (best Slam result)
Average games dropped: 5.5 (2006); 5.4 (2007)
Average ranking of players faced: 33.8 (2006); 30.4 (2007)
Numbers on Maria's 2008 AO run:
Sets lost: 0
Opponents in top 10 defeated: Jelena Jankovic, Ana Ivanovic, Justine Henin
Other notable opponents: Elena Dementieva, Lindsay Davenport
Average games dropped: 4.6
Average ranking of players faced: 32.4
Numbers on Lindsay Davenport's AO 2000 run (I consider this a part of the 2000-2009 decade):
Sets lost: 0
Opponents in top 10 defeated: Martina Hingis
Other notable opponents: Anna Kournikova, Sarah Pitowski, Julie Halard-Decugis, Jennifer Capriati
Average games dropped: 5.4
Average ranking of players faced: 47.8
Numbers on Serena's 2002 USO run:
Sets lost: 0
Opponents in top 10 defeated: Lindsay Davenport, Venus Williams
Other notable opponents: Dinara Safina, Daniela Hantuchova
Average games dropped: 4.1
Average ranking of players faced (sans Morariu): 27.3
Based on this, I would have to say Serena's was the most dominant as her opponents were on average higher ranked than Maria's or Justine's, though all 4 slams were equally impressive.