Of course, I have to say Sesshomaru! Haha, but he's not the only one...
The king of heroes: Gilgamesh. He's not human because he's technically dead and brought back as a powerful spirit to fight in the Holy Grail Wars. He has to have a certain amount of mana from his master, also known as a mage, in order to be in the world of the living.
And here's another spirit for you. Yami Bakura, or otherwise known as the Thief King. I find him so awesome!
One of the lesser known demons, Kougaiji. He so cool and powerful. And his little sister I just adore. Rin (from Inuyasha) and Lirin (from Saiyuki Reload) are just too cute!
And of course, Espada number 6, Grimmjow! We can include Ulquiorra with this as well ;) @InVinsybll