Chesnaught @ Black Belt
Ability - Overgrow
The "Knight" of the three starters.
I kept Low Sweep and gave Chesnaught a Black Belt for extra damage. Spiky Shield protects from most attacks (good for Sucker Punch blocking). Seed Bomb is a powerful Grass-Type attack, besides the STAB bonus. Sadly, Chesnaught cannot learn Leaf Blade in any possible way, so Seed Bomb is the best you can do. Dragon Claw is good for pretty much everything (except Fairy).
Chesnaught has good defense and great attack, and decent speed. Only thing it can't do much against with this moveset is Fairy.
Weaknesses: Flying, Psychic, Fire, Ice, Poison, Fairy
Resists: Water, Electric, Grass, Ground, Rock, Dark
Malamar @ Scope Lens
Ability - Contrary
Psycho Cut and Night Slash's increased critical hit rate along with the Scope Lens help take down anything, and Malamar's high Attack and Defense continue to increase damage. Thunderbolt is the only special attack in the moveset, but Malamar's decent Sp. Attack can make those Thunderbolts HURT. With Malamar's ability Contrary, Superpower instead increases Attack and Defense, not lower. Malamar's higher than average speed can take down most near-level opponents quickly.
Malamar would be harder to work with if not for its typing. It has few weaknesses, but no resistances. My Malamar with this moveset took down half of Diantha's team in OHKO's.
Weaknesses: Bug, Fairy
Talonflame @ None/Rocky Helmet
Ability - Flame Body
Flamethrower is a good Fire-Type move, and good for damage and speed. Fly is a two-turn move, and is HM-taught. Acrobatics is good for the No-Item moveset, along with Steel Wing. Steel Wing has lower damage, but does double-damage to Ice and Rock types, and Rock does increased damage on Talonflame. If you go with Brave Bird, you could go either way and take Steel Wing or Roost. Roost heals for half of Talonflame's max HP, and loses its Flying-Typing for the rest of the turn, making Rock do less damage and have Ice be a resistance.
Talonflame is good for any team, really but it depends on how you play.
Weaknesses: Rock, Electric, Water
Resists: Grass, Fighting, Bug, Steel, Fairy, Fire
Aigislash @ Leftovers
Ability - Stance Change
Sacred Sword's Fighting type is odd for a Ghost-type, but it bypasses all stat changes and hits for good damage. Night Slash hits for decent damage, and attacks with high Crit. chance. Iron Head is one of the best Steel-Type offensive moves, hitting for decent damage and possibly making the enemy flinch. King's Shield is a good blocker, with increased priority and blocking Sucker Punches. King's Shield also lowers Attack of any opponent that makes contact (except a few moves).
Aigislash is pretty awesome overall. Mine took down Mega Gardevoir with one Iron Head ten levels down. It also has a bunch of resistances.
Weaknesses: Ground, Ghost, Fire, Dark
Resists: Flying, Rock, Bug, Steel, Grass, Psychic, Ice, Dragon, Fairy
Immune: Normal, Fighting, Poison
Tyrantrum @ King's Rock
Ability - Strong Jaw
Tyrantrum gets Rock Slide late (around level 60-65), so I went with Rock Tomb until the E4, then I farmed Victory Road for Rock Slide. Rock Slide does moderate damage, and has a chance to make the target flinch, and King's Rock increases that chance, though Rock Slide does have slightly lower accuracy. Earthquake does massive damage to your enemy, but also hurts your teammate (in a double battle), unless they are a Flying-Type or have Levitate as an Ability. Crunch does increased damage with Strong Jaws, and can decrease Defense. Dragon Claw is good for fighting other Dragons (Tyrantrum has decent speed), and can do good damage by itself.
Tyrantrum is overall a good sweeper, with good Attack and Defense, but has many weaknesses.
Weaknesses: Fighting, Ground, Steel, Ice, Dragon, Fairy
Resists: Normal, Flying, Poison, Fire, Electric
Blastoise @ Blastoisinite
Surf and Waterfall are decent moves by themselves, having 90 and 80 Power, respectively, and are used outside of battle to travel. Dark Pulse is good for Mega Blastoise, as the gained ability Mega Launcher damage for increases pulse and aura moves (Such as Aura Sphere or Dark Pulse) by 50%! Ice Beam is a good move overall, and with Blastoise' increased Sp. Attack, can do incredible damage.
Blastoise was never my favorite starter, but is good for Sp. Attack and Sp. Defense.
Weaknesses: Grass, Electric
Resists: Steel, Water, Fire, Ice
This is probably one of the best teams I have ever had. It is balanced, and the types cover each other very well. I use other Pokemon for most HM's and Rock Smash, but I think Fly, Surf, and Waterfall are good enough moves by themselves to use in battle.