Emiya, Rider Kintoki, Tamamo cat or Rama

It depends on which enemies you are facing, but Rider Kintoki is arguably the best choice. His Animal Communication skill, combined with Waver's awesome NP boosts, means that you'll be able to fire off his NP quite frequently, especially if you equip him with an MLB Sumo. Because he's a welfare servant, I'm assuming you have him at NP5, so he has huge NP damage potential. Plus, he'll get great star gen through the use of Long-Distance Dash, his NP's Quick performance boost, and his double Quick cards with high hit counts. He also benefits from the Rider class's high star absorption rate, so he's the rare servant that can generate and then make good use of stars, which will synergize quite nicely with Waver's critical damage-boosting skill.