blitzcrank with good aim, pulls, stuns, and they both do decent damage. blitz can either initiate with stun(when pushed back to turret) and use grab as a chaser, or pull them away from their turret and stun them. that on top of yi's dps is pretty deadly.
Annie is another good choice since her combo nearly kills most people, and yi can just finish them off.
jax makes for a double melee, but his stun is pretty useful as well.
kassadin slow, silence(to prevent harassment), and ability to burst someone down helps yi.
malphite has a weak slow, but softens other carries and initiates well.
nasus has a very strong slow if leveled and can lower armor with spirit fire, but will take away last hits.
sion has a good burst combo and can stun quite often.
tristana has a slow and can knock enemies away or towards yi.
twitch sets up nice ganks and has a pretty devastating slow.
warwicks ult sets up nice ganks and he makes everyones atk spd higher, and can tell you who to gank alongwith him, but steals your lizard/golem buff by jungling.
ashe has a reliable slow and her arrow stun is quite long.
amumu has a stun as well and also softens up tanks.
mundo really softens up tanks and has a slow with nice damage.
eve isn't too great of a partner but she has a stun and alot of burst.