Steven. He's stronger, more durable, and his shields can deal with most of Star's attacks. The only issue is Star's more hax spells like time freeze, but Death Battle has said they go for who would win more times than not instead of who has the best chance to win in a handful of very specific ways. Star never used the spell again even against powerful enemies so it would make no sense to hand her the victory with that. Then again they could always just scale Star to Omnitraxious being a multiverseal entity that controls space time which would be a really lame way of handing Star the win considering he never does anything that powerful in combat.