1. He has the most personality
2. He has the most versatility
I like all of Slaughterhouse, but Ortiz is easily my favorite.
He's the most emotional member. He can make make me laugh one minute and pump my adrenaline in the next, often times in the same verse. While he's the most humorous of the group, he never becomes a clown, as many of his songs are darker and realistic. He can be a comedian without it blemishing his hardcore image. Considering his roots, he's also one of the hardest and probably pushed through more hardship than the other members of the group.
His flow is very versatile. Budden and Crooked, for example, (great rappers, btw) are consistent, but they're too consistent to the point where they sound the same like they have a formula they use for every song. Ortiz sounds different in each of his verses.
He can go offbeat or completely change his flow in the middle of a song, whereas the other members stay the same throughout the whole verse. He can deliver it both fast and slow, both lighthearted and viciously hardcore.