Urahara Kisuke...the most mysterious character in Bleach...he is the most smart person, he made the Hogyoku, he "created" the hollowfication and about his powers, he hasn't lost a fight, even thought he hasn't fought much but when Renji asked Urahara to help him during his training with his Bankai he said that "My bankai isn't suited for training"(clue #1) and the Royal Guard member with the healing pool (can't remember his name) said that Urahara copied that to make his own healing pool that we see during Ichigo's Bankai training so we know that he has been in the Soul King's realm(clue #2)
So there, i think i made my point!=D
I also like Aizen, Isshin, Ryuuken and generally all the "veterans" of Bleach!Also Byakuya...i like his style and his "never lose my dignity and honor" attitude of his!Those are the kind of characters i like in animes!(Like Madara, Hashirama and Itachi in Naruto)