Starmie is great for getting BPs out of first train (21 battles), it's very easy to catch her, you teach her Surf, Ice Beam, Thunderbolt and Psychic and give her Expert Belt. She'll 3:0 most opponents. As simple as it could be. Darmanitan is good as well, basically you just want to ohko opponents. Something else to deal with Sturdy Pokemon that give Starmie trouble. Priority might be nice.
Dragons are generally great Pokemon because they can strike anything very hard with Draco Meteor or Outrage. Easiest recipe for decent team is Dragon + something resisting Dragon's weaknesses (Steel type) + third Poke to clean up.
Buy EV items (Power Anklet etc.) with earned BP - they will be a huge help with both breeding and training. After that, buy items like Choice Band and Life Orb and you'll be set to create competitive team.