Kindness, attentiveness. Making you feel as though you're the most important thing in his life. Not too clingy, but not too unreliable. Unreliable may be exciting for a while, but in the end just gets irritating. Sense of humour. Not selfish.
Well, of course physical attraction is first. I want someone tall, handsome, and with dark hair. Then of course, personality is more important - someone who is loving, funny, reliable, comforting, intelligent, compassionate, successful/motivated, and kind.