I like how Swashbucklers seem to get a good amount of Swashbuckler companions to join your party.
HOWEVER, the class I would personally pick based on their first mate would be Witchdoctor or Musketeer. Both classes are very fun to play, and I just love Musketeers.
Witchdoctor's first mate (the goat monk) ends up being one of the most powerful companions in the entire game, while the Musketeer's first mate (the crane) ends up being one of the most hardiest companions in the entire game. Virtually same talents, but a privateer class, with a LOT more health and defense/resistance, and a little less damage.