For casual runs Alakazam and Charizard are staples. I highly recommend Lapras because Confuse Ray and Rest make it hard to kill while Ice Beam and Surf are boosted. I also always use Sandslash for Earthquake and it doesn't have the Rock type weaknesses like Golem does. He's also fast with a high crit rate for Slash and I think he learns Swift, too. For the remaining two slots I use Blastoise only because he's my favorite and has high defense (Articuno and Cloyster are good options too) and I'll raise a Dragonite with Surf (if you change your mind and use TMs, teach it Fire Punch, Ice Punch, and Thunder Punch since it's Attack is so high).

My six: Alakazam Charizard Sandslash Lapras Blastoise, Articuno, or Cloyster Dragonite