Hippo leads arn't too good now because of TrickGars. Trust me, I used one, he's my fav poke.
But, if you're still going to use Hippo, go with Thunder Fang. It covers Gyarados. It 2HKOs with Intimidate, and you may get a parahax too if you're lucky xD. But the Gyara might think you'll switch, so...