Bleach, Naruto, or One Piece?
Now, I don't know specifically why, but I would assume that they're called the Big 3 because they were the most popular(Among Kids/Teenagers I think) in the last decade... not counting Yu-Gi-Oh I guess =P
Now could I be wrong? Yes, but hey, at least I'll have people correct me now before I continue to be incorrect elsewhere...
Oh yeah, I'm indifferent to all 3, I'd much rather watch a Super Sentai from that period(I said SENTAI you perv!)
Dekaranger, Boukenger, or Abaranger over the Big 3 any day! =)