As a starter team, I would reccommend this:
If you chose Piplup:
Budew, Rotom (can be found in Old Chateau at night in a TV), Gligar
(can be found at Route 206), Bidoof (as HM slave) and Buizel. When you
made it to Veilstone City, dump Buizel (or Floatzel) for Houndour (can
be found at Route 214).
So when fully evolved, you now have:
(Water - Steel), Roserade (Grass - Poison), Rotom (Electric - Ghost),
Gliscor (Ground - Flying), Houndoom (Fire - Dark) and Bibarel (Normal -
Water). If you don't want an HM slave, replace Bibarel with Machop (will
later evolve into Machamp (Fight)) or Abra (will later evolve into
Alakazam (Psychc)). I don't reccommend dumping Bibarel though because
you will need Surf and Strength in the Distortion World, and also Rock
Climb to get there.
If you chose Chimchar:
Rotom, Gligar, Bidoof and Vaporeon (Eevee can be obtained in Hearthome
City and if you use a Water Stone on it i
t'll evolve into Vaporeon. If you don't want Bibarel get an Abra.
If you chose Turtwig:
Turtwig, Starly, Rotom, Machop, Abra, Bidoof. If you don't want Bidoof replace it with Buizel.
As a stronger team, I would reccommend these Pokémon:
Infernape, Torterra, Gliscor, Roserade, Floatzel, Rotom, Staraptor,
Alakazam, Machamp, Luxray, Gyrados, Crobat, Steelix, Vespiquen,
Heracross, Ambipom, Drifblim, Gengar, Medicham, Gallade, Gardevoir,
Bronzong, Rapidash, Houndoom, Blissey, Spiritomb, Garchomp, Dragonite,
Snorlax, Lucario, Milotic, Weavile, Uxie, Azelf, Mesprit, Giratina,
Palkia, Dialga, Vaporeon, Jolteon, Flareon, Espeon, Umbreon, Leafeon,
Glaceon, Togekiss, Magnezone, Yanmega, Dusknoir, Porygon2, Porygon-Z,
Scizor, Electivire, Magmortar, Absol.