My favorite Bleach character is Renji Abarai, so I'll open the discussion with Zabimaru. Zabimaru and Renji are both proud, fierce, love to fight, but hate to loose.

Renji Abarai's zanpakuto is Zabimaru, which means "Snake tail". Zabimaru's shikai command is roar (or howl in Jap. versions). Zabimaru is one of only a few that can be seen, and actually speaks. Better yet, the baboon and snake can speak separately. Zabimaru is a *nue (a baboon with a snake tail), which are supposed to be bringers of misfortune and illness. The baboon is the stronger more dominant of the two symbiont personalities. However the snake is more volatile, and his a very nasty streak.

Zabimaru's shikai form:

Transforms into a segmented blade with a bagger like protrusions on each of the segments. Each segment is connected by a stretchable thread, which makes Zabimaru more like a whip. Zabimaru is limited to three consecutive attacks before having to be brought back together for recovery. Renji can use his spiritual power to separate the segments, surrounding an opponent with them, allowing him to execute a one-shot omnidirectional attack called higa zekko (broken baboon's fang). However this attack is damaging to Zabimaru and leaves Renji defenseless. Zabimaru can stretch out to incredible lengths, which can give Renji an added advantage of distance during a battle.

Zabimaru's bankai form:

Hihio Zabimaru (Baboon King Snake Tail) and transforms Zabimaru into a massive version of its shikai. In this form Zabimaru resembles a skeletal snake. Zabimaru gains many more segments, each segment is much larger than in the shikai form, and each segment is very tough and more difficult to defeat. It also gains a skeletal snake-like head about the size of a small car. During bankai Zabimaru's segments are held together by Renji's spiritual power. This allows him to separate and reattach them at will. If a segment is damaged, Renji simply detaches it and reforms Zabimaru. In bankai form Renji usually catches his opponent with the snake-head of his weapon and smashes his opponent into the ground with it, or Zabimaru can bit the opponent.

Zabimaru also gains a special technique known as Baboon Bone Cannon (hiktsu taiho) after Renji becomes more adept with using his bankai. Which fires a dense blast of spiritual energy from Zabimaru's mouth. He can also use a much more powerful higa zekko, where all the segments detach and sharpen using spiritual energy.

Although Renji is my favorite character in Bleach, his early bankai is not. Yes it is highly impressive, much more so after he becomes more skillful with it. Perhaps it would be more so, if not for the odd costume change he has upon bankai. What's with the fur wrap (short cape like cowl)? It looks rather lame to me, but...

Once Renji learns more control, he and Zabimaru gain more impressive technique's.

I thought Renji and Zabimaru did incredibly well in their first battle. Yes he lost, brilliantly, but he held his own for awhile, and showed the stuff he and Zabimaru are made of.

*A nue is a legendary creature found in Japanese folklore. It is described as having the head of a monkey, the body of a raccoon dog, the legs of a tiger, and a snake as a tail. Due to its appearance, it is sometimes referred to as a Japanese chimera.