Personally, I think the Psychological Class is the best: I beat Sonny 2 with that class in way less than 2 days. I played like 6 hours the first day (call me a no-lifer) and about 4 the next. The second best class I would say in the Hydraulic class.
Dunno why Krin said the Biological class is the best....Or maybe he said: "This class is recommended for Beginners".
Whatever... But if you are starting to play Sonny 2 for the first time, I highly recommend the Psychological class.