Cultures are very imbalanced. In stock, Berzerkers is by far the best. Combat bonuses make a huge difference. Take 120 aggressiveness and defensiveness too, and...
Merchants comes a distant second, if you take 115 maintenance aptitude characteristic. The idea is to spam the Berzerker with lots of ships...
PvK Balance Mod is your best bet for stock-like game racial balances. It alters the cost of racial characteristics as well, which are quite imbalanced in stock...