Thunderbolt, Will-O-Wisp, or Psychic.
Thunderbolt effectively deals with Crasher Wake, plus some of Cyrus' and Cynthia's teams. Psychic is general coverage, but does deal quite nicely with Cynthia's Roserade. Both are also quite good at taking down random Trainers.
I would go for Will-O-Wisp in the Battle Tower, as that burn could potentially be game-changing. If you're threatened by physical attackers, Drifblim can really help take them down.
Hope I helped!